Refactored themes into simple build

## Motivation

I wanted to add some more colors for ReST, and I got annoyed that I had to hunt to find the single existing rule for ReST within a large file.  

I also noticed that the rules for Markdown were not all together; they were split up into two different parts of the theme.

We can do better!


## Changes

- Split up the rules for each language under `src/`
- Replaced all Solarized color values with `__«VARIABLE»__`
- Added a simple `build.sh` script that concatenates the files for each language together, then replaces all the variables with their Solarized color values


## Benefits

- Less intimidating/tedious to work with source
    * Easier for contributors to to add a new language
    * Easier for contributors to spruce up their favorite language
    * No more looking up color values; intuitively write `__RED__`, and you’ll get Solarized red
- Language rules are now always contiguous in the built `.tmTheme` file (no more “Markdown rules … other rules … *more* Markdown rules”)
- Nonstandard colors—normally unwanted, but occasionally useful—are now obvious