[PATCH 0 of 3] perform regex replacements in display only

[David Champion <dgc@xxxxxxxxxxxx>]

This patch series adds the ability to replace subjects and mailbox names
in the index according to a regular expression.  Only the display
value is modified; the internal value for search and limit purposes
is unchanged.  This reuses the existing mutt code for spam engine
detection.

This capability allows you to programmatically hide certain patterns
in mailbox names ($status_format) or subjects ($index_format), if that
information is not useful.