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.