mu4e: add custom variable for list of visible header flags - take 2
Merge pull request #178 from nicolasavru/recursive_maildirs

* improved recursive maildir support
Merge pull request #180 from abend/master

Customize visible header flags take 2.1
* mu4e:	small cleanup
* mu4e: copy address at point with c / C-u c in message-view
* mu4e: allow starting mu4e in the background
* mu4e: add background-starting to the FAQ
* mu4e: make 'a' the action key; fixes issue #181
* mu4e: small doc fixes