printExport support for fish-shell

This is, basically, just an upstream update of boot2docker/boot2docker-cli#260. I just didn't used @kevinbin commits 'cause I couldn't find his repo anymore to merge with my remote.
Yeah, this was removed on purpose. :smile:

See the discussion at https://github.com/boot2docker/boot2docker/issues/585, especially:
> there's no harm in exporting the same value again :)
> 
> (although the explicit "unset"s are nice, so keeping those and expanding
them to include all possible vars as either an "export XYZ=..." or an
explicit "unset XYZ" would be superb)