Add functionality for finding an online archive for a ML message

* mu4e/mu4e-lists.el (defgroup mu4e-lists): New group.

* mu4e/mu4e-list-archives.el: New file.
(defgroup mu4e-list-archives): New group.
(defcustom mu4e-list-archives-user-actions): New customizable
variable for locating online archive.
(mu4e-list-archives--get-recipients): New helper function for
getting all recipients of a mail.
(mu4e-list-archives-resolve-debbug): New function for resolving
archive url on debbug systems.
(mu4e-list-archives--resolve-namazu): New helper function for
resolving real archive url from namazu search page.
(defcustom mu4e-list-archives-resolve-namazu-search): New
customizable variable for disabling namazu resolution because it
incurs a url fetch.
(mu4e-list-archives-resolve-mailman-namazu): New function for
getting the namazu search url for mailman systems.  This is as
close as possible without fetching any url.
(defconst mu4e-list-archives-actions): New constant for builtin
supported mailing lists.
(mu4e-list-archives-resolve): New function to resolve the concrete
url to the mailing list archive.

* mu4e/mu4e-actions.el (mu4e-actions-browse-list-archive): New
command for browsing the online archive of a mailing list.
(mu4e-actions-kill-list-archive): New command for putting the url
to the online archive of a mailing list onto the kill ring.

mu4e/mu4e-lists.el

@@ -84,6 +84,9 @@
      ("zsh-users.zsh.org"                                     . "ZshUsr"))
   "AList of cells (MAILING-LIST-ID . SHORTNAME).")
 
+(defgroup mu4e-lists nil "Configuration for mailing lists."
+  :group 'mu4e)
+
 (defcustom mu4e-user-mailing-lists nil
   "An alist with cells (MAILING-LIST-ID . SHORTNAME).
 These are used in addition to the built-in list `mu4e-mailing-lists'."