Frag should override applyTo with a default value

[lihaoyi]

Otherwise you have to do the following to inherit from Frag:

object View extends Frag{
  val view: Frag = ???
  def render = view.render
  def applyTo(t: dom.Element) = t.appendChild(render)
}

Not the end of the world, but more annoying than it should be. applyTo is always just going to append the rendered frag to the builder element