image/jpeg: add a jpeg option to disable chroma subsampling

[urandom]

It seems that golang's jpeg encoder only supports downsampling. It would be nice to be able to fine-tune what kind of subsampling is used when encoding jpegs, or alternatively disable downsampling when using a large quality value.

This is a continuation of: https://groups.google.com/forum/#!topic/golang-nuts/VJY87VQpgws

[rsc]

/cc @nigeltao @robpike

[nigeltao]

Yeah, it's a valid bug, but I don't know when I'll have time to fix it.