throttling-feature added

[kunukn]

Hey, now that 1.6.0 is released, could you prepare a PR with your code from http://codepen.io/kunukn/full/bgjzpb/? I'd merge it and release as 1.7.0. I guess the throttle function could be simplified a bit, because we won't be using leading/trailing options.

To test this. You use this in the app.js

signaturePad = new SignaturePad(canvas, {
  throttle: 16 // x milli seconds
});

The throttle.js is 34 lines and is from Underscore. Well-tested and production ready.

I tried this 22 lines example which could be used instead.
https://remysharp.com/2010/07/21/throttling-function-calls

I tried this 12 lines example, but could not get it to work correctly with my Codepen example. Not sure why.
http://sampsonblog.com/749/simple-throttle-function

[szimek]

@kunukn Big thanks for this feature, especially that it took so long to add... I've just released 2.1.0-beta.1 with it and if nobody finds any issues, I'll release it as 2.1.0 soon.

BTW. I set throttling to 16 by default.