… Sourcegraph

I'm working on adding syntax highlighting for Cue to Sourcegraph - and almost had it working today but found out that we're missing the `fileTypes` declaration here (which is what we use in [syntect_server](github.com/sourcegraph/syntect_server) to detect file types.)

Once this is merged, I should be able to get syntax highlighting of Cue files on Sourcegraph.com working sometime next week (and in our next release.) :) 

Note that this is just following the TextMate grammar / JSON schema format, nothing here is unique to Sourcegraph:

```
            "fileTypes": {
              "description": "this is an array of file type extensions that the grammar should (by default) be used with. This is referenced when TextMate does not know what grammar to use for a file the user opens. If however the user selects a grammar from the language pop-up in the status bar, TextMate will remember that choice.",
              "type": "array",
              "items": { "type": "string" }
            },
```

If there are other common extensions than just `.cue` that should be considered Cue language files, we should include them here as well.

Signed-off-by: Stephen Gutekanst <stephen@sourcegraph.com>