Merge pull request #7 from ayman/master

defgroup customization
Fix default action text
Fix some docstrings
Add parent maildirs info and clean up code.

This adds parent maildirs info. Parent maildirs can be propertized now.
Propertize function has been changed to only recive 1 param (a plist) that
contains all the maildir information. Some variables have been clean up and
some functions renamed.
Remove unused variable
Update docs
[Issue #10] Add custom function for action str
Fix compilation warnings
Adding defcustom for mu4e-maildirs-extension-action-text to allow for
nil correctly.  As per issue #11.