Allow asynchronous transform functions

I was looking for a way to generate a jade mixin file for svg defs allowing me to write `+icon('clock')` instead of having to write the svg tag every time. I managed to do this using your `afterTransform` callback with the data provided but because it's a synchronous function call I couldn't be guaranteed the generated jade template existed when my template gulp task was being called. I might have been able to do this by using writeFileSync but it didn't seem this best way.

I added the config option `asyncTransforms` which defaults to `false` so it won't break the current api.

Let me know what you think. :smile: 

Thanks!