audit: Progressive JPEGs as part of opportunities

WebPageTest includes their own audit for whether sites are shipping down progressive JPEGs (images that are delivering with scans of increasing quality). This is something that may be interesting to explore as a Lighthouse loading audit.

**Proposed description:** Uses Progressive JPEGs
**Proposed help text:** Delivering JPEGs with progressive decoding allows progressive delivery of images as a series of "scans" where each scan increases quality. This can improve perceived performance over "baseline" JPEGs, which display images as data arrives, one line at a time.


WPT presents this in an opportunities-style view that looks a little like this:


http://blog.patrickmeenan.com/2013/06/progressive-jpegs-ftw.html