Add ui-variables

This will add the `ui-variables` to the Styleguide. They're the same as the official list from the [docs](https://atom.io/docs/v0.152.0/theme-variables#variables), but as a "living" styleguide. It shows the actual values next to the variable name.

![screen shot 2014-12-04 at 2 28 55 pm](https://cloud.githubusercontent.com/assets/378023/5294142/2b94b11e-7bc3-11e4-8a83-df8259236ef4.png)

![screen shot 2014-12-04 at 5 00 43 pm](https://cloud.githubusercontent.com/assets/378023/5295235/268b4908-7bd7-11e4-8892-75127bd1b33c.png)

Should also be useful for theme designers to test if all variables are set.