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)
It _seems_ like what this function does is compute the appropriate "extract" parameters for convert such that the image will be clipped to the document bounds. Is that right?

If that's right, I'd suggest something like `getExtractParamsForDocBounds` or something. I'd also suggest adding a 1-2 sentence comment explaining what exactly is being calculated. (Yes, yes, I know I should heed my own advice and add a comment to getPadding... apologies for being hypocritical.)