Add option to store stripped DOM at OnNewState

I made this small change to the CrawlOverview plugin in my local fork. I thought it was not practical for debugging that the plugin stores the browser's DOM instead of the vertex's strippedDom. And apart from debugging, I think the current behaviour is counter-intuitive: it is the stripped dom that determines whether we're dealing with a new state, so for me, that is the dom that should be stored.

The default behaviour is still the same, users can use the setter method on the plugin to change the behaviour. Up to you to decide whether you want to merge this :)