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.