Add screenshot functionality for long-term provenance

[NateLanza]

This should be a function that takes a DOM node to screenshot and arguments related to when to screenshot. A config should be provided to choose which actions trigger screenshots. Either default include all or exclude all and provide a list of action keys which are exceptions to the overall rule. Once ephemeral nodes are added, they should not be included in the count before next screenshot.

[NateLanza]

To discuss: are we OK with only png screenshots or do we need jpeg and webp support? Png will work on all browsers for sure, jpeg and webp aren't guaranteed to have browser support. For now I'm going with only png.