- org-mu4e.el: add "mu4e:"-link type and handlers to org-mode
- Makefile.am: add org-mu4e.el to package
- mu4e-hdrs.el: register handler functions for server data *outside* header
mode, so they can be used independently (and we can link to messages even
when mu4e is not active already)
- mu4e-view.el, mu-server.1, mu-proc.el, mu-cmd-server.c: support viewing
messages by msgid (rather than only by docid); document it
