[MERGE] Add Mutt as a supported email client

[Keir Mierle]

This adds Mutt as a supported email client. It also
rearranges the listing order of the mail clients in various
places to be alphabetical for easier scanning.

Suggested config options for Mutt to facilitate faster
sending, to be added to ~/.muttrc:

set autoedit
set edit_headers

Keir
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