Added precompileCallback support

Decided finally to clean up a bit and upload my take to it. This is an attempt to add `precompileCallback` support, as a response to #264.

Related to #248 and #265 as well, possibly related to #242.

(I have to sheepishly admit that I didn't spend enough time to test this one out throughoutly. Please see if it solves the autoprefixer and/or other needs. :P)