Add Gremlin language syntax

[PhilStollery]

A graph language that would be nice to see in colour.

[gusbemacbe]

@PhilStollery

I rewrite what @ldct said in other issues:

We can review and merge PRs with language implementations but won't be able to actually implement all the requested languages.

Please see http://highlightjs.readthedocs.io/en/latest/language-requests.html for more details.

[joshgoebel]

Asked and answered, thanks @gusbemacbe.

Rather than leave these issues open to languish forever (as we have in the past) we're going to start closing them (to clean up the issues list).

I've moved this request to the wish list:
https://github.com/highlightjs/highlight.js/wiki/Grammar-Requests