Add circular crop?

[k034b363]

Is your feature request related to a problem? Please describe. A few times recently it has seemed like cropping to a circle would be useful, but I'm not sure how hard it would be. One specific example involves X-ray images of seeds inside of a Petri dish. The dish is very bright, so being able to crop to everything inside, even if it means just zero-ing out some padding around the outside to keep displaying a rectangle, would I think help some downstream analysis.

Describe the solution you'd like A separate function like the current crop, but that takes a center point and radius as input for cropping.