Try to read prompt as JS whenever possible

Compile CS to JS and cache it if your prompt uses CS

This saves about 50 ms
Conditionally load CoffeeScript

This saves about 50 ms
Extract helpers
Get rid of COMPILED_DIR variable

Makes prompts easier to test
Update tests to use their own compiled prompt location
Merge pull request #14 from Impromptu/improve-coffeescript-performance

Improve coffeescript performance

Cuts ~100ms out of prompt generation