callback function on common interactions

[cornhundred]

Is your feature request related to a problem? Please describe.
Clustergrammer-GL is being used by Clustergrammer2 and information needs to be passed from Clustergrammer-GL to the Clustergrammer2 widget. We need to enable callback functions to be passed to Clustergrammer-GL. These can be used for instance to: update the widget state on mouseover.

Describe the solution you'd like
Have callback functions as arguments to Clustergrammer-GL. Have these callback functions run on specific interaction events (e.g. click, mouseover).

Describe alternatives you've considered
I have not considered alternatives.

Additional context
This is going to be useful for Clustergrammer2 widget functionality.