LookML

Add support for LookML, the language component of [Looker](http://looker.com).

I expect the majority use of LookML is in private repositories. Looker generally pushes directly to private repos on GitHub since this code can contain proprietary information. Nevertheless, there's quite a bit of LookML floating around in public:

- https://github.com/snowplow/snowplow/tree/master/5-analytics/looker-analytics/lookml
- https://github.com/wilg/headlinesmasher.lookml
- [A search for LookML files on GitHub](https://github.com/search?q=dimension+extension%3Alookml&ref=searchresults&type=Code)