Easier way to define a fcc directory

In the common case that a user only has one FCC (save outgoing mail in
the Mail directory, it is now possible to simply configure a string
such as "Sent" in the notmuch-fcc-dirs variable. More complex options,
depending on a users email address, are possible and described in the
variable customization help text.

The whole function notmuch-fcc-header-setup has been cleaned up a
little while working on that.

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