Node placeholder API

ASDisplayNodes now have an overridable method -placeholderImage that lets you provide a custom UIImage to display while a node is displaying asynchronously. The default implementation of this method returns nil and thus does nothing. A provided example project also demonstrates using the placeholder API.
this is awesome for a first pass at ASTextNode placeholders ✔︎

once we've got the primary placeholder API hammered out and ready to go, let's revisit this and make it an inset for each shaded row of text, à la Paper's text placeholders.  that might require adding plumbing to ASTextNodeRenderer but will be drastically nicer-looking