Add maildir segment to agnoster theme

This adds an extra segment to the agnoster theme, showing whether the local Malibox contains any unread email; useful for mutt/offlineimap users, it works as long as the $MAILDIR env variable is set, and pointing to the Maildir folder (which contains the actual individual mailboxes).
![Screen Shot 2013-04-15 at 15 10 03](https://f.cloud.github.com/assets/387628/380998/a0ca7564-a5d6-11e2-8bc4-1bac09d726e1.png)