feature request: 🎨 option to set diff draw color.

[cramatt]

It seems like a useful feature to provide the option to set the color for the image diff. For example in my project we are comparing red screens... so red is difficult to distinguish.

The function which uses to color is https://github.com/mapbox/pixelmatch/blob/master/index.js#L147-L152.

has anyone approached this feature before? Do you have strong opinions on how it should be done. I'm willing to take on a PR to help!