Implement clip to document as image post processing

This is ready to go, and is required by https://github.com/adobe-photoshop/generator-assets/pull/315

This adds the options to calculate the area to extract if you want to clip to the document bounds (or any clipTo bounds you want to use)
The `extract` parameter of the `settings` object should be documented in the `Generator.prototype.savePixmap` function.

Incidentally, so should the `padding` parameter. I'll add that to master -- I don't know why it wasn't there before.