Update Stripe importer to rename tax field

This reflects the current CSV output from the "Export" button.

patreon: Exclude tax from full amount.

This makes the balancing logic in entry generation work the way we want.

patreon: Update importer for this month's change.

Literally all they did was change the title of the amount column
(maybe it doesn't assume USD anymore?).

patreon: Update importer for recent patron changes. RT#12747

All they did was change a couple of column names.

github: Update to match fiscal host reporting.

In general the fiscal host reporting isn't as nice as the project reporting,
so I hope someday we can revert this change, but until then that's what
we're using.

hooks.ledger_entry: Clean up whitespace in strings.

The hook now ensures it does not output whitespace that could be
significant to Ledger, either because it's a newline or an
account-amount separator.

tests: Pass amount to Template.render as string.

render is required to convert the string to Decimal for historical design
reasons. Passing the amount as a string verifies this behavior.

hooks.ledger_entry: Improve date handling.

Check that we have the date field used in the payee line, and not just
'date'.
Allow other date fields to be None since they may not be used by the
template.

nbpy2017: Report item rate names and final unit prices.

This keeps more of the RBI calculation in the templates, where it belongs.
Template logic has become more capable since this importer was first
written, which makes this change practical.

tests: Use yaml.full_load when available.

Per <https://msg.pyyaml.org/load>;.

csv: Support importing squared CSV spreadsheets.

See the test comment for more rationale.

ledger_entry: Support NEWLINE tokens.

Necessary to run on Python 3.7.

ledger_entry: Only try to balance entry amounts when necessary.

This gives template authors the ability to write entries that
balance something other than the "main" amount.

ledger_entry: Support string tests in amount expressions.

This is useful for setting amounts based on imported strings like country,
Eventbrite ticket type, etc.

ledger_entry: Improve error reporting for amount expressions.

This is going to become more important as amount expressions become more
capable.

eventbrite: Sales report can include currency.

It's a selectable column that I apparently failed to include in my last
report. Keep USD around as a default for convenience.

benevity: Remove noop header copy.

The name of the header was "Payment Reference," so this never worked.
And now the information doesn't even exist in 2019 reports, so don't
bother with it.

benevity: Version the current importer.

The report format has changed slightly in 2019.

nbpy2017: Rename _RATE constants to _RBI.

This is a nonfunctional change that's just meant to better document
what these values actually represent.

nbpy2017: Handle complimentary tickets.

The conference is offering these to people who proposed talks this year.

importers: Catch and handle CSV parse errors in can_import.

import2ledger bailed on one of these exceptions when trying to import
a recent XLS file that had a null byte on a line.

outfile: New module to take over functionality from config.

With the concept of the general "output path" going away, it doesn't make
sense for Config to have an open_output_file method.

config: Separate initialization from argument reading.

This lets us fix the bug in __main__ where the exception handler tries to
call my_config.error but my_config wasn't successfully created.

config/hooks: Date hooks configure themselves.

Broadly, this change gets date-specific configuration out of the Config
class, and into the date hooks. Following this pattern, we can have more
specific configuration for more hooks without growing a bunch of custom
methods on the Config class.

There's still more cleanup to be done to finish the job, but this commit is
about as big as I can stand, so this will serve as a good checkpoint.

dynload: Pre-configure objects and skip unconfigured ones.

Now that all importers and hooks receive the configuration object, they can
tell us at initialization time whether or not they actually have enough
configuration to be useful. Initialize them immediately on loading, and
only use importers and hooks that are actually configured.

config: Remove unused method from_section.

The configuration is migrating to have sections set aside for specific
importers and hooks, so using the configuration this way no longer makes
sense.

There's still more cruft to clean from the Config class along the lines of
this change, but that will have to wait until more of the migration is done.

tests: Separate test_hooks_add_entity file.

All the hooks are going to get their own test files over time.

importers: Initialize importers with the user's configuration.

This will enable us to import from sources that require additional
configuration, like web APIs with authentication tokens. This required a
complete revamp of the importer API, but it makes it more uniform with the
hook API, which seems like a plus.

brightfunds: New importer.

This commit adds infrastructure to treat XLS files like CSV files, and
import them using the base classes that already exist for that.

hooks.ledger_entry: Look up templates dynamically.

If there's a 'ledger entry' key in the entry data, use that value as the
name of the template to load. Thanks to this, nbpy2017 could collapse
multiple importers into one.

Otherwise, build a default template name based on the importer source, and
try to use that.

All the configuration names now end with "ledger entry" instead of starting
with "template". This makes it clearer what they're for, in case we
support other kinds of output templates in the future.

I ended up changing the names of some of the importers so the default
template name was nice, rather than specifying template names for all of
them, to reduce the amount of name discrepancies across the codebase.

hooks.ledger_entry: Bring in all Ledger-specific code.

This includes the old "template" module, plus the associated
template-loading code from config.

config: Make get_section a public method.

As hooks and importers both require more configuration, they're gonna need
to have lower-level access to configuration settings.

hooks.ledger_entry: New hook to output Ledger entries.

This is roughly the smallest diff necessary to move output to a hook.
There's a lot of code reorganization that should still happen to bring it
better in line with this new structure.

config: Add open_output_file method.

Make this functionality accessible to hooks.

README: Document Benevity disbursement importer from 1b1e2d0.

With this, we've finished paying down our recent technical debt.