.emacs.d/site-lisp/wtk_notmuch.el: Add a "mentions" search

I watch a number of GitHub repositories that I'm moderately interested
in.  I don't read *all* of the resulting messages, but it's nice to
have them in my email archives so they'll come up in future searches.
However, sometimes messages that @mention me may get lost in the
deluge.  To highlight them, I tag them (using the X-GitHub-Reason
header):

  $ for FILE in $(grep -lr '^X-GitHub-Reason: mention' .); do
  >   MID=$(cat "$FILE" | sed -n 's/^Message-ID: <\(.*\)>.*/\1/p' | head -n1);
  >   notmuch tag +mention -- "id:$MID";
  > done

This commit adds a saved search [1] that grabs messages mentioning me
from my inbox, so notmuch's hello page will show a count, and I can
read them over before removing inbox tags.

[1]: http://notmuchmail.org/emacstips/#index12h2