split out actual combining logic into a module

[yocontra]

the gulp plugin should just feed the css into this module and receive out new css. this will clean up the code base a lot and let other people use the code for other build systems

[ben-eb]

👍

Seems like this project and https://github.com/frontendfriends/node-combine-mq (https://github.com/frontendfriends/gulp-combine-mq) have similar goals.