Add subjectrx command to replace matching subjects with something else.

Add subjectrx command to replace matching subjects with something else.

This lets you define regular expressions-replacement pairs for subject
display.  When a Subject: matches the regular expression, the replacement
value will be displayed instead in the message index.  Backreferences
are supported.

This is especially nice for simplifying subjects that are overly wordy,
such as mailing list posts (with [Listname] tags, etc), mail from
ticketing systems or bug trackers, etc.  It lets you reduce clutter in
your mutt display without altering the messages themselves.