Fix #8, Fix #20 => v2.3.0 potentially

+ Now supporting the generation of background variations. These are defined in `colors.json` in the `backgrounds` property. Fixes #8.

+ Rewrote process of getting external color schemes, it extendable if you follow the templates outlined in [Monocyanide Colorscheme](https://github.com/Centril/sublime-monocyanide-colorscheme). It also supports the generation of background variations for the color schemes being generated.

+ Added [Twilightcyanide Colorscheme](https://github.com/Centril/sublime-twilightcyanide-colorscheme) which is an adation of the Predawn Twilight colorscheme. Fixes #20.

+ Recommended to release as version 2.3.0.

+ TODO, @lefoy: The images for [Monocyanide Colorscheme] & [Twilightcyanide Colorscheme] are wrong, update them with Mac OS X, the image for the sidebar file icons needs to be updated.