Jison Atom package

Adds syntax highlighting to Jison files in Atom, as well as on Github.

Thanks to, and based in part on, the following projects:

1. Bison improved plugin
2. Bison plugin
3. Lex/Flex plugin

Thanks to the following contributors:

1. Nate Whetsell
2. Dan Freeman

Contributions are greatly appreciated.
Please fork this repository and open a pull request to add snippets, make grammar tweaks, etc. Thx!