Avoid sorting by nil
Merge pull request #429 from sabof/default-to-descending

* mure: headers sorted by "nil" at first; avoid this.
* mu4e: some doc updates
* mu4e: unbreak address display, removing toggling
* mu4e/org-mu4e.el (org-mu4e-store-link): Add capture template keywords

For other email links (e.g., wl, mh, gnus), a number of keywords are
defined in org capture templates.  This makes these keywords available
for use in captures from mu4e.

http://orgmode.org/manual/Template-expansion.html

Inspiration from Roelof Burger
https://groups.google.com/d/msg/mu-discuss/4sffZofVKAY/aVTdVYoVyiUJ
Revert "* mu4e: don't add rogue \n where re-editing"

This reverts commit 5ab30cbc0998f209f3037255c5a3182e5151b75c.