Add RegexFilter

The RegexFilter can work on any line of the header, and add tags based
on a group in the regex.  The ListMailsFilter is now based on it.

A use case is email sent to XYZ-team@example.com.  You could do:

    [RegexFilter.1]
    header_field = To
    regex_string = (?P<team_name>[a-z0-9]+)-team@example.org
    regex_group = team_name