support tiny plugins

[rbuels]

Somebody should be able to just write something tiny like:

class MyTinyPlugin {
   configure(pluginManager) {
   }
}

in a script file or script tag somewhere and have that be a valid JBrowse plugin.

Maybe this is already possible and somebody just needs to prototype or document how to do it.

[cmdcolin]

Fixed by #2798 will want to add a little documentation or example probably