Previously -B implied -x; now any of the valuation flags do.
This avoids a bug where print -V of a transaction with an implicit
commodity conversion would convert only some of its postings to value.
Also, more valuation tests.
-V (and -X) now respects a report end date set with -e/-p/date: when
choosing the valuation date, similar to hledger 1.14 and Ledger.
This means that -V/-X aren't exactly like either --value=end or
--value=now. The "Effect of --value on reports" doc has been extended
accordingly, and much of it has been reworded and made more accurate.
