emacs: allow functions in notmuch-show-stash-mlarchive-link-alist

Some archives may use a more complicated scheme for referring to
messages than just concatenated url and message-id. In particular,
patchwork requires a query to translate message-id to a patchwork
patch id. Allow functions in notmuch-show-stash-mlarchive-link-alist
to facilitate this.

For example, one could use something like this for patchwork.

(lambda (message-id)
  (concat
   "http://patchwork.example.com/patch/"
   (nth 0
	(split-string
	 (car (last (process-lines "pwclient" "search" "-n" "1"
				   "-m" (concat "<" message-id ">"))))))))