Support height and xdim image options

RFC @jamesbrink — __DO NOT MERGE YET__

This is a quick shot at adding options to the image request such as `height` and `xdim`.  It should be noted that `height` is only applicable on 1D barcodes and `xdim` on 2D.  Furthermore, I'd like to determine a better name for `xdim` (what Barby refers to it as), any thoughts?