Preprocessor modules

[Fauntleroy]

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
• 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.

[Fauntleroy]

@pushred review pls

[pushred]

One thing I'm seeing in this branch is I no longer get this in the console:

Waiting...[SOLIDUS] Server running on port 8080

I only get:

Waiting...

[pushred]

Also, please rebase. The caching in dev is making testing a pain.

[Fauntleroy]

@pushred do you like now?

[Fauntleroy]

@pushred I am going to go ahead and merge this into master this week. I can't just let this sit around.

[Fauntleroy]

This is now running live on http://keithurban.net and doing just fine. While it's not quite as performant as I had hoped, it's still an improvement over our past implementation.

[Fauntleroy]

https://github.com/SparkartGroupInc/solidus/issues/58