Lexer for Inform 7

This pull request adds a lexer for Inform 7 as requested in #1725.
The lexer is at: https://github.com/PogiNate/Sublime-Inform.

I opened an issue on the repository to clarify the license situation: [PogiNate/Sublime-Inform#2](https://github.com/PogiNate/Sublime-Inform/issues/2).

Rather than remove it, I suggest adding an entry for Inform 6 to `languages.yml`.