emacs: add `notmuch-show-stash-mlarchive-link{, -and-go}'

emacs: add `notmuch-show-stash-mlarchive-link{, -and-go}'

* emacs/notmuch-show.el

  (notmuch-show-stash-mlarchive-link-alist):
    New defcustom of type `alist' (key = name, value = URI),
    containing Mailing List Archive URI's for searching by Message-Id.

  (notmuch-show-stash-mlarchive-link-default):
    New defcustom, default MLA to use when `notmuch-show-stash-mlarchive-link'
    received no user input whatsoever.  Available choices are generated using
    the contents of `notmuch-show-stash-mlarchive-link-alist'.

  (notmuch-show-stash-map):
    Added keybinds "l" and "L" for `notmuch-show-stash-mlarchive-link'
    respectively `notmuch-show-stash-mlarchive-link-and-go'.

  (notmuch-show-stash-mlarchive-link):
    New function, stashes a URI pointing to the current message at one
    of the MLAs configured in `notmuch-show-stash-mlarchive-link-alist'.
    Prompts user with `completing-read' if not provided with an MLA key.

  (notmuch-show-stash-mlarchive-link-and-go):
    New function, uses `notmuch-show-stash-mlarchive-link' to
    stash a URI, and then visits it using the browser configured
    in `browse-url-browser-function'.

Based on original work [1] by David Edmondson <dme@dme.org>.

[1] id:"1327397873-20596-1-git-send-email-dme@dme.org"