To match +beancount/previous-transaction's docstring: this command
should jump to the start of the transaction/directive at point first,
before jumping to the previous one. Now it does so.
It should also return nil if it fails.
+beancount/next-transaction and +beancount/previous-transaction would
formerly skip timestampped directives, e.g.
2016-09-15 * "Foo" ""
Expenses:Y 779.00 DKK
Assets:X:Checkings
2016-11-16 balance Assets:X:Checkings 8,417.58 DKK
2016-12-13 * "Bar" ""
Assets:X:Checkings 2,100.00 DKK
Income:Z
The `balance` directive would be skipped over. This commit changes that
to include them. Perhaps the commands should be renamed to
+beancount/{next,previous}-entry to better reflect my intended purpose
for them?