Add theme color, contrast, and saturation settings

Initial work towards making themes adjustable by making certain Less variables defined by config settings and used in themes.

![theme-settings](https://cloud.githubusercontent.com/assets/671378/5002209/f528ea86-69b2-11e4-9ac9-8fa01d6530ce.gif)

@simurai couple questions for you:

* What do you think the preset colors should be?
* What do you think the preset contrast percentages should be?
* What do you think the preset saturation percentages should be?

Depends on https://github.com/atom/atom/pull/4153
Can we specify these in the theme metadata?