add screen mode (idea by jostber <http://github.com/jostber>)

To use it, set the following variables:
export MUTTJUMP_USE_SCREEN=yes
export MUTTJUMP_SCREEN_SESSION=foobar

start mutt inside screen session:
  screen -S foobar -t Sent mutt -f =Sent
  screen -X -S foobar eval "screen -t INBOX mutt -f =INBOX"
(optionally start more mutt instances)

screen window names are generated from the mailbox path using
the function MUTTJUMP_SCREEN_WINDOW_NAME_MANGLE(mbox_path),
which defaults to basename(mbox_path).