emacs: possibility to customize the rendering of tags

This patch extracts the rendering of tags in notmuch-show to
the notmuch-tag file.

This file introduces a `notmuch-tag-formats' variable that associates
each tag to a particular format. This variable can be customized
thanks to the work of Austin Clements. For example,

  '(("unread" (propertize tag 'face '(:foreground "red")))
    ("flagged" (notmuch-tag-format-image tag "star.svg")))

associates a red foreground to the "unread" tag and a star picture to
the "flagged" tag.

Signed-off-by: Damien Cassou <damien.cassou@gmail.com>