filter: Support matching on To and Cc headers
Always keep recipients list

If an email arrives at the system and the system cannot determine why the
mail arrived (ie. a system email was not found in the To or Cc headers), the
recipients list should be maintained.

This is useful for generating collaborator lists as well as filtering by the
To and Cc (Addressee) headers.
Merge pull request #529 from greezybacon/feature/to-cc-filter-match

Add addressee to filtering system

Reviewed-By: Peter Rotich <peter@osticket.com>