Would like to upstream some options: diffMask, drawAA

[zol]

We use pixelmatch at Chromatic and I've been maintaining our own fork with some extra functionality. I'd much prefer to upstream these changes and wanted to feel you out first to see whether you'd be open to merging them. They are backward compatible out of the box and come with tests. I believe they are aligned with pixelmatch's ethos of remaining small, fast and flexible.

The functionality is behind two new options:

• diffMask - If true, output will be a mask rather than a complete image. false by default.
• drawAA - If true, output will contain aaColor pixels for those detected as anti-aliased otherwise they won't be included in the diff output. true by default.

I can explain the reasoning behind needing these options in case it's unclear.

[mourner]

Hey @zol, this sounds great — PRs welcome! If I understand the context behind these options correctly, would it be simpler to automatically disable drawing AA pixels when diffMask is true (so that the mask is different pixels only, which what I'd expect too) instead of adding drawAA option?

[zol]

Awesome, I'll cook one up without the drawAA option as soon as I have a sec.

[tmeasday]

@mourner - I've taken over this and created a PR here: #73

[mourner]

Released as v5.1.0.