Allow to do a lossless jpeg to jpeg optimization using mozjpeg only

[flozz]

Add choices for JPEG optimization:

• Guetzli + MozJPEG

  • Always reencode JPEGs using Guetzli and then use additional MozJPEG (lossless) optimization

• MozJPEG

  • Always reencode JPEGs using libjpeg and then optimize losslessly with MozJPEG

• MozJPEG lossless

  • If the input image is already a JPEG: only perform a lossless optimization using MozJPEG (ignore the quality parameter)
  • If the input image is not a JPEG: encode the image using libjpeg (Pillow) and then optimize it with MozJPEG (lossless)

Related issue: https://github.com/flozz/yoga-image-optimizer/issues/15