[PATCH 1/2] emacs: Add notmuch-combine-face-text-property-string
[PATCH 2/2] emacs: possibility to customize the rendering of tags

These patches are the first of an upcoming series whose goal is to
integrate notmuch-labeler into notmuch. See the following for more
details: https://github.com/DamienCassou/notmuch-labeler

- These patches depend on
id:1360013822-6562-1-git-send-email-amdragon@mit.edu
"notmuch-combine-face-text-property improvements" by Austin Clements.

- This series does not have any unit-test to make it smaller and more
  amenable to comments. I will send a patch when requested.

- Patch 1/2 just add a convenient function to call
  `notmuch-combine-face-text-property' on strings.

- Patch 2/2 introduces `notmuch-tagger-formats', a list of pairs (KEY
  FORMAT) to format a tag matching KEY using a special format.
  Currently, an example of such a list is:

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

  to set the unread tag to be red and the flagged tag to have a star
  picture attached. This variable can be customized, thanks to the
  work of Austin Clements.

- Patch 2/2 also provides 3 pictures, all in SVG and all directly
  embedded in the elisp source code as proposed by Austin Clements and
  Tomi Ollila.


Compared to v2, I took care of Austin Clements' comments by
simplifying embedded SVG pictures and by reorganizing some functions
and their comments.