use ipython display formatter directly for missing _repr_mimebundle

[morlic]

Also fix #232, #284

I am not sure this will be fully backwards compatible, IPython code mentions that _repr_mimebundle_ is there since 6.x and the DisplayFormatter should take care of rendering all registered mimetypes, but still.

My problem was that graphviz Source objects expose only _repr_mimebundle which is not used by the metakernel so I had to wrap that, but this looks like a simple enough fix that worked for me. DisplayFormatter can be configured, perhaps the kernel kwargs should be passed to it, not sure.