Saving track configs in stateful URL.

Tracks in JBrowse view are saved/loaded in/from URL.

Behavior is enabled by default in JBrowse app. However, in order to turn it on in other apps (GenCover), JBrowse directive needs **keepState: true flag**. 

 
ok, I'll look into gencover to see the config examples.

Basically, the problem is that addTrack accepts final JBrowse object only, which is just fine with all data types except the genome data item, which needs some trickery before it is added (purging refseqs, reloading them). Now, to get the genome track to render, I used public API function and simulated the data object.

I too noticed a need for a better solution. I'll think a bit about it.