Add style guide rules about modules and requires

@spicyj @benkomalo @chrisklaiber 
Everything under `third_party` should be considered third party, even if it doesn't have a `node_modules` entry.

`typeahead.js`, at the moment, is not considered third party. But I'd actually really like to move it into `third_party`, since it's lint blacklisted, and I really don't like having first party files in our lint blacklist.