JPEG → JPEG optimization: keep the original image if the output image is bigger than the input one

[flozz]

When doing a JPEG to JPEG optimization, we should never have an output image bigger than the input one.

• If the guetzli+mozjpeg reencoding produce a smaller image → keep it
• else, try a mozjpeg (lossless) optimization alone, and if it produces a smaller image → keep this one
• else, just remove metadata and keep the original encoding.

Issue #43 must be resolved before this one

Related issue: https://github.com/flozz/yoga-image-optimizer/issues/14