New port mail/py-afew: Initial tagging script for notmuch mail

Its basic task is to provide automatic tagging each time new
mail is registered with notmuch. In a classic setup, you might
call it after 'notmuch new' in an offlineimap post sync hook.

It can do basic thing such as adding tags based on email headers
or maildir folders, handling killed threads and spam.

In move mode, afew will move mails between maildir folders
according to configurable rules that can contain arbitrary
notmuch queries to match against any searchable attributes.

WWW: https://github.com/afewmail/afew

PR: 222629
Submitted by: Sebastian Schwarz <seschwar@gmail.com>
Reviewed by: mat, koobs
Differential Revision: D12520