historical: Move normalize_rate and _pretty_rate to base Formatter.

This reduces the number of method overrides to help readability,
and gets rid of the annoying format_rate/pretty_rate distinction.

historical: Fix the rate ordering with two dates.

Darnit, I wrote the tests first, and I wrote them right, and then I
mixed up the ordering in the code, and somehow I convinced myself
the code was the right and the tests were wrong. But no, I had the
tests right, this is really what we want. This gets the output to
follow the examples from our bookkeeping documentation.

historical: Always format rates with the same precision.

When we format a rate as a price, we don't know how much precision
is "enough" to do the conversion, because we don't know what's
being converted to. As a result, we may (=will almost certainly)
end up formatting the rate with different precision on the cost
date vs. the price date, and that causes Beancount/Ledger to fail
to make the connection between them.

Using a constant of 6 is enough to make the current test for
"enough" precision pass, so just do that for now. This might need
further refinement in the future.

historical: Swap Ledger and Beancount formatters in the class hierarchy.

This makes sense for a couple of reasons:

* The Beancount formatter has "less features" than the Ledger formatter, so
this is a more "logical" organization of the hierarchy anyway. Note how
this eliminates the need for the BeancountFormatter.__init__ override to
turn off Ledger features.

* Any future work will probably be focused on the Beancount formatter, so
this reduces the amount of code you have to understand and hold in your
head to do that.

historical: Beancount can handle commas in amounts.

And having it looks nicer, is more consistent with our historical
books, is less code for me, and is no more trouble for the user.

historical: Normalize class instantiation for formatters.

This will make them easier to extend with different price/cost rates,
and to add a Beancount formatter.

historical: Introduce --output-format option.

Beancount output will be added here.

historical: Add a setting to denominate Ledger conversions.

This makes conversion output easier to add to ledgers directly.