Changelist:
- Add init_filters() func as an entry point when filters are reloaded
from a file. This is a good place to put any code that should be
loaded once instead of on a per-message basis.
- Added helper functions for moving messages and checking if a message
has any matches in a list of strings.
- Created local object caches for normalized keyword lists and mailbox
folder IDs. This puts the data closer to where it is needed, and
removes the need for passing 'self' around when filtering.
- Added filter rule to catch unactionable email.
