feature: allow blocking and css manipulation within 'end' event

my [stylus-lemonade](https://github.com/mikesmullin/stylus-lemonade) plugin necessarily makes two passes when pre-processing the stylus markup: 1) once during various calls to `define()` functions from within the `*.styl` stylesheets, and 2) again at the end once all the sprites that will be added have been added, so i can go back and update things like final dimensions and positions in the output sprite images.

the second pass is done via `emit('end')` event emitted during `stylus.render()`. this change request is to allow blocking during that event, and manipulation of the css result.