Integrate notmuch-fcc mechansim

I have gone wild and added a defcustom "notmuch-fcc-dirs".
Depending on the value of that variable we will not do any
maildir fcc at all (nil, the default), or it is of the format
(("defaultsentbox")
 ("full name <email@address>" . "Work/sentbox")
 ("full name2 <email2@address2>" . "Work2/sentbox"))

The outbox name will be concatenated with the message mode
variable "message-directory" which is "~/Mail/" by default.

Signed-off-by: Sebastian Spaeth <Sebastian@SSpaeth.de>