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**. 

 
I will need to add it in gencover, but it's not an issue.
For consistency, we should add `:` to [`data:genome:fasta:gc`](https://github.com/genialis/genapp-jbrowse/pull/32/files#diff-3f83e924b7e35e33380904338d6d0081R108)