register_renderable is not ideal

[shashi]

I think the right way to go about this is to provide a trait:

WebIO.renderable(x::T) = true # and WebIO.renderable(x::Any) = false

Then the backends are to be hooked up to always use WebIO.render when this trait is true.

Best way to do this is have backends depend on WebIO.

[twavv]

This is better now™. We have a @register_renderable macro and should only get better with the way that IJulia and things are moving.