Snapshot comparison improvements

[zepumph]

While using the snapshot comparison tool to check on phetsims/sun#631, I had the following thoughts on improvements:

• Add a button next to "Start Snapshot" that only reruns the items that had an error on last snapshot
• Speedup the tool by either loading more sims in more iframes at a time, or kicking off the loading of the next iframe once the first iframe is loaded and is being fuzzed.
• Button to clear all snapshots images that you have clicked up.

Tagging @jonathanolson to see if he has qualms about these thoughts, or wants to add more.

[jonathanolson]

All of that sounds great. Feel free to rebuild the interface/UI/handling completely, or make whatever changes. It's in a pretty rough state.

[zepumph]

@chrisklus just had such an amazing idea that I'd like to try out

• Have a "manual mode", likely for just a single sim, where the sim loads, and you put it into a state, and then "take snapshot", then have another button that reloads to let you put the new version into that same mode before pressing again "take snapshot", it may be best as a second html page using much the same code. Thanks @chrisklus!