Add optional callback.

[jordanabderrachid]

No description provided.

[mnort9]

Hey Jordan, thanks for contributing. I previously had an optional callback in an older version, but deprecated it. I didn't think anyone would want their app logic waiting on an analytics tool.

Can you give an example of how you would use the callback?

I'm hesitant to add it back because I may be adding additional arguments to the track functions to support future platform features. If the callback is added, all additional arguments will break backwards compatibility. It's not the end of the world to have to bump the major version, but I want to make sure adding it back is warranted.