emacs: Add auto-tagging for replied messages.

emacs: Add auto-tagging for replied messages.

Add `notmuch-message-mark-replied', a function for automatically tagging
replied messages with user-defined tags. The tags (which can be either
added or removed) can be customized with the customization variable
`notmuch-message-replied-tags'. This is a simple list of strings. Any
string prefaced with a "-" will be removed; any string prefaced with a "+"
(or neither "+" nor "-") will be added.

This adds a new file notmuch-message.el, for functions which target
message mode (and in the future, notmuch-message mode). Based on some
conversation, notmuch-message.el will probably end up subsuming
notmuch-mua.el, but until we figure out exactly how we want to do that,
they will remain separate files.

Edited-by: Carl Worth <cworth@cworth.org>: Remove trailing whitespace
and add newline at end of file.