Be able to view custom elements in the local DOM view.

[sachinhosmani]

Right now the + buttons that let us see the local DOM view of an element are shown only for Polymer elements. Considering how Polymer elements are just custom elements, it should make sense to be able to provide the same facility to custom elements.

We can still hold the color code of blue for Polymer elements and something else for the rest.

[sachinhosmani]

@nevir, would you mind looking into this?
DOMJSONizer gets all the DOM information we need to show custom elements this way.
I guess we should set a flag for the UI (element-tree) to know that an element is a custom element.
We should still have the isPolymer flag to show Polymer elements' differently (color, properties and model).
Thanks.