Add ability to load a ?hubURL= from URL bar

[cmdcolin]

This is a demonstration of loading a UCSC trackhub from the URL bar using &hubURL

It may not be the final word on this but there a number of "interesting" concerns it has to handle

• The hubURL works without any config.json being loaded. In fact, it currently bypasses any config loading, and just creates an empty config, empty session, and loads the hub as a sessionConnection
• To accomplish the above, the hubURL goes through an entirely separate path in the "SessionLoader" (session loader normally processes the URL parameters and such) and as a result of some of the choices here, this means that currently hubURL is not "composable" with other existing URL params e.g. you can't use &loc and &hubURL unless i reimplement &loc for &hubURL, etc.

It's possible that some of the above design choices could be reconsidered, but I wanted to have a demo of hubURL for the UCSC<->JBrowse interop for PAG :)