[not ready] Show tm2

re: #794 -- Now you can see `.tm2` and `.tmsource` files in the file browser but cannot navigate into them.

Potentially in conflict with @samanpwbb's improvements re: https://github.com/mapbox/mapbox-studio/issues/755, 'openable' files are now highlighted (e.g. when saving, directories are emphasized; when browsing for a project, the `.tm2` and `.tmsource` files are emphasized):

**Saving:**
![screen shot 2014-09-09 at 6 33 22 pm](https://cloud.githubusercontent.com/assets/3952537/4211567/5811a4a2-388b-11e4-83ea-81ad78043400.png)

**Browsing:**
![screen shot 2014-09-09 at 6 38 08 pm](https://cloud.githubusercontent.com/assets/3952537/4211569/5c232a2a-388b-11e4-9470-7f26bdad5b56.png)