Message filters plugin

This plugin provides the ability to move messages to other folders based
on a set of filter rules. Suggestions and feedback are encouraged.

- A filter management page where users can add/edit/delete filter rules and
  manage filter options. It can be accessed from the mailbox options
  section of the options page. A filter requires 3 possibly 4 options:

     - pattern:    The string that if found triggers an action
     - target:     The part or parts of the message to search for the pattern
     - action:     One of three things: move; flag; delete;
     - time frame: The age of the message can be combined with a pattern or
                   used alone within a filter rule
     - desination: The destination folder if action is set to "move"

  Filters are executed in the order they are listed on the management page.
  They can be rearranged with the "up" and "down" links in the list. It is
  possible to execute both "flag" and "move" or "delete" filters on the same
  messages by creating two filters and placing the "flag" action filter higher
  in the list.

- There are 2 additional options a user can set on the management page:
     - Auto filter the INBOX whenever it is selected
     - Show a link to the filter management page on the main Hastymail2 menu

- Filters can be applied to any folder using the "Filter" button in the message
  controls of the mailbox page.

I plan to add support for filtering on a specified header field soon. Other
suggestions for features are welcome.

Jason Munro
jason@hastymail.org
