Make signature_pad an event emitter

[UziTech]

This is an issue to track #353 (comment) probably for v4

[JamesCatt]

Just wanted to +1 this. In particular, I'm hoping to be able to execute a callback if/when the user hits the edge of the canvas.

My use case is to try and make it usable by blind users—i.e., alert them via a sound or aria-live announcement if they run off the edge with a stroke.

Happy to contribute if it helps move this along.