Add LESS support and change opts.transform signature

OK, so this is kind of messy still but it's late here and I wanted to get feedback. Basically, I moved the existing functionality into `css.js` and created `less.js`, and I call the appropriate one based on the extension of the entry file.

I also changed `opts.transform` to be a simple string in, string out function, but we can change that if you want. In fact I guess we should if we don't want to rule out async transforms.

All the `test/` files are in progress and are just there to see what's going on. You can switch between the two entry files in `test/index.js` to see the different behavior. If you are good with these changes I will clean things up and write some proper tests before merging.