Add a basic build system

[faultyserver]

The XML-based syntax for themes is not very conducive to DRY code, and it's easy to get lost with all of the verbosity around the actual important code.

Having an abstraction layer will do a few nice things:

1. Reduce chance of copy-paste errors by using variables for colors.
2. Simplify adding new rules by using a cleaner syntax.
3. Simplify creating theme variants by allowing multiple build targets to be given.

[faultyserver]

I think the easiest build system would be a Makefile that runs a Ruby script to generate the proper XML.

To match the .sublime-syntax structure, the abstraction layer would make sense as a .yml file.