Add crop option

[kurtextrem]

Cropping uploaded avatars probably makes sense.

[thomask]

Definitely. I'd be happy to merge a pull request if you'd add this functionality.

[kurtextrem]

Currently thinking of embedding https://github.com/fengyuanchen/cropper into the preview div.

[thomask]

Crossed our minds here as well. Would be great if we could make this an optional feature (with optional dependency). We'd be happy to merge a pull request 👍

[kurtextrem]

#7 🎉