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.
One thing we seem to have lost here is the automatic silencing.

In other words, if a variable is already set correctly, no message was output.

Do we want to preserve that functionality?