Allow specifying additional files types & grammar files for syntax highlight from within the repository

Please add support for specifying additional files types & grammar files for syntax highlighting in gitlab's code view from within the repository itself.

Why is it useful?

1. bleeding edge support: language developers hosting the very latest bleeding edge implementation on a gitlab instance (or gitlab.com) could have their own latest language code highlighted correctly with all bleeding edge language features which might have an altered syntax. Getting changes into rouge and then back into the actual gitlab instance takes time, which makes it not very suitable for correct display of bleeding edge features

2. barely used short-lived mini languages support: some small super domain specific experimental language projects might not be significant or long-lived enough for inclusion into rouge, but the language developer might still want to provide basic syntax highlighting support for the gitlab hosted project

Ideally, gitlab's code view should pick up such highlighting additions from the branch the file currently shown is from, inside some common path (e.g. a config file .gitlab-syntax.yml inside the root of the tree).

The format with which you will ask the language developers to use doesn't really matter, although I imagine a full-blown rouge lexing script might be problematic due to the security considerations of running it on the server / gitlab.com. Adding support for some alternative regular expression-based format like Textmate or the Atom text editor use it might be required to implement this nicely.