Merge.hs: added support for USE flag renaming into hackport

Now to build xmobar we mostly need somthing like this:
$ hackport merge xmobar -f-all_extensions,+with_threaded,+with_utf8,with_iwlib:wifi,with_alsa:alsa,with_xft:xft,with_datezone:timezone,with_dbus:dbus,with_mpd:mpd,with_inotify:inotify,with_mpris:mpris

Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>