historical: Introduce --output-format option.

Beancount output will be added here.

config: Accept a source date for displaying rates.

For output formats that can show both a cost and a price,
this will be used to distinguish them.

config: Nicer determination of default currency for converting/signing.

base defaults to USD more as an API restriction than anything else, so avoid
using it as a default setting. Instead, use the user's books denomination
or locale setting.

config: historical command accepts just two currency arguments.

This shows the rates between two currencies without converting a specific
amount.

historical: Add a setting to denominate Ledger conversions.

This makes conversion output easier to add to ledgers directly.

config: Nicer implementation of date parsing.

Keeps less local state through the function, to reduce the risk of that
state getting inconsistent.

config: More flexible date parsing.

* Accept partial dates, filling in the current year and month as needed.
* Accept more separators.