Combine only if mimosa build -o

[fjorgemota]

Hey.

Just a note to improve yet more this library: It will be nice if there is an option (maybe?) to only combine CSS when optimization flag on mimosa is enabled.

This occurs because combining CSS is, for almost all the times, an optimization for loading time, and because this does not makes sense to combine all the times.

Well, this is just a suggestion. Feel free to discuss. :)

[dbashford]

I could add an option and default to to false.

But I just added source maps. Let me know if that solves any issues you think you might have with how this works now.