That new green shell, fixes #1640

The blue/purple color of Shell files was close to that of Golang and the two could likely be paired together often. This updates Shell to a bright green [inspired by the terminal](https://github.com/github/linguist/issues/1640#issuecomment-61296290) and @bkeepers :green_heart: 

Fixes #1640 

cc @samlambert @arfon