Add a custom protocol handler for JBrowse Desktop

[garrettjstevens]

There is an example here of how to register an electron app as a handler for a custom protocol. If we regester e.g. the jbrowse:// protocol, we could use a link like jbrowse://something to open JBrowse Desktop from a browser or another app. One use for this could be an "Open in dekstop" menu item in JBrowse Web.