Added jump plugin

Hey there.  This is my first attempt to contribute to an open source project, so if I'm going about it wrong, please forgive me.  I've been using this little utility in bash for a while and have found it to be tremendously useful.  I decided to rewrite it in fish, and got a little carried away with beefing it up a bit, but I think that it's still very lightweight and useful.  Hopefully others will feel the same.  Cheers!