JPEGStream: add progressive mode option

Progressive jpegs have several very useful advantages over baseline
jpegs:

* Slightly smaller for the same quality (3-7%)
* Load much faster in most browsers.

This adds the `options.progressive` option to the
canvas.jpegCreateStream() function.
Merge pull request #309 from iFixit/progressive-jpegs

JPEGStream: add progressive mode option