Preprocessor modules

This should keep preprocessors from catching on fire, but requires a significant amount of work from developers to understand what's going on. The changes include:

- Using child processes to run preprocessor methods via [worker-farm](https://github.com/rvagg/node-worker-farm)
- Adding a timeout for long running preprocessors
- The ability to escape infinitely looped preprocessors
- Using node modules for preprocessors instead of plain javascript files
- Libraries like Sugar, Underscore, etc are no longer included by default
- Additional libraries can be used if the user installs them

My main concern here is that people writing preprocessors will need to know how to write node modules... which isn't necessarily a bad thing, but another small hurdle to jump over on the way to making a site. Ideally, users will never have to make preprocessors (especially once we have better helpers in place), but we will certainly need to add better instruction, nonetheless.

This is also a massively breaking change, so we'll need to bump the version up pretty aggressively.