Project 2022-12-055, task 2: Add search-engine functionality to the e-mail client

[BjarniRunar]

The goal of this task is to flesh out the required back-end API methods and deliver a text-mode tool which is capable of monitoring a set of mailboxes, adding their contents to a search index, and then allow the user to perform search-related activities, such as tagging/untagging and searching.

Milestone(s):

• Watch mailboxes or IMAP servers for new mail
• Add discovered mail to the search index, and apply filters which auto-tag/untag mail based
  on manual rules or bayesian content analysis
• Augment the list view in Urwid to allow tagging/untagging/searching
• Document how to use relevant moggie APIs from the CLI or as a web API
• Store OpenPGP session keys in metadata index (stretch goal)