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: Add a setting to denominate Ledger conversions.

This makes conversion output easier to add to ledgers directly.

config: More flexible date parsing.

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