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).

Unfortunately just changing the scope name here won't work. `script/download-grammars` regenerates this file when it runs and will undo this change. (In fact, the best way to add new grammars to this file is to run `script/download-grammars --add https://github.com/PogiNate/Sublime-Inform`.)

And if we aren't using this grammar anymore we should just remove it.