Add constraint info to parcats click/hover events

[jonmmease]

This PR adds a new constraints key to the event data emitted by parcats click/hover events. The keys are dimension indices or color, and the values are category values.

When clicking on a path, there will be a key for each dimension. When clicking on a category rectangle, there will be a key for the dimension the category belongs to. The color key is only added if:

1. A color array was provided
2. The click was on a path or on a category with hoveron: 'color'

[etpinard]

Nicely done @jonmmease ! 💃

[jonmmease]

Thanks!