Refine renderer implementation with options

[geotrev]

Right now upgraded-element is tied to omdomdom. While that's a perfectly fine renderer, it would probably be helpful to disentangle the concept of the renderer from the base component.

In this world, there would be a separate package called upgraded-renderer which could have a few options in it:

• "omdomdom" -> renders with omdomdom diff
• "snabbdom" -> renders with snabbdom diff

And possibly others.

[geotrev]

Done!