Add onCompleteCallback option

[inorganik]

I'm submitting a...

[ ] Bug Fix
[x ] Feature
[ ] Other (Refactoring, Added tests, Documentation, ...)

Checklist

• [ x] Test your changes
• Followed the build steps

Description

Adds a new option onCompleteCallback to the options object which will get called on completion of the animation. For backwards compatibility (and convenience), if a callback is passed to the start method, it will replace whatever was set in the options.

Also updated the demo and added several tests

Does this PR introduce a breaking change?

[ ] Yes
[ x] No

[inorganik]

Resolves #302