Add IMAP keywords support

Add IMAP keywords support

This change introduces IMAP keyword support.

* index_format gets %g, %G, %J and %Gx to display them
* pattern gets g to search/match imap keywords
* colors.c gets index_tag/index_tags to color them
* OPS gets modify-labels to edit them
* tag-transforms/tag-formats/hidden_tags can be display them in more sexy manner

Most of the code is shared with the notmuch backend.

Upstream: https://dev.mutt.org/trac/ticket/3210
Closes neomutt/neomutt#225