Add "wildcard" Markov search/don't copy input seed word punctuation/capitalization exactly

Certain operations (just the nikkybot/ai “markovXXX” search
commands for now) can use wildcard placeholders to indicate
that any word can appear there, for better flexibility
(though DB query performance may be less).  This required
reworking the PostgresMarkov class, which may introduce a
bunch of bugs (hopefully not, of course…)

Also, PostgresMarkov's sentence generators no longer reuse the
exact punctuation/caps of the input seed word(s) given; it
chooses its own from the text corpus it trained on.