Ability to select a circle area instead of rectangle

[snario]

Would love this added feature.

[heyman]

Sounds like a nice feature, however I think it would be non-trivial to implement it on top of the current code base, since you would have to use some other technique to create the transparent "hole" (probably some CSS clipping/masking). Unfortunately, I don't have the time to develop this myself.

With that said, I would be happy to accept a pull request that would add this feature while keeping the code neat.