Add Fish Shell support.

Carelessly thought Linguist was still using [Pygments](https://bitbucket.org/birkenfeld/pygments-main/), so went ahead first and [committed](https://bitbucket.org/birkenfeld/pygments-main/pull-request/422/add-fishshelllexer-class-to-support-the/diff) a `FishLexer` in the main repo @ BitBucket. Later found out Linguist uses Textmate grammars so I set out to write one for Fish. Fortunately, I found [this gorgeous relic](https://github.com/l15n/fish-tmbundle), tested it with [LightShow](lightshow.githubapp.com/) and added the relevant references in this commit. 

Happy Github Fish browsing!