Support vega and vegalite mimetypes directly

[rgbkrk]

Rather than making an extension on the jupyter notebook, I'd like to see vega and vegalite supported out of the box in the classic notebook. JupyterLab, nteract, hydrogen, and commuter are all supporting it out of the box. Since the mimerendering customization requires reinitializing all outputs on the page after load, I'd like to see vega get built in by default.

[jpivarski]

As it is, developers are tempted to use _repr_mimebundle_ for JupyterLab, because it recognizes the "application/vnd.vegalite.v2+json" MIME type directly, and _ipython_display_ in Jupyter Notebook as a fallback.

In fact, it's more than a fallback because _ipython_display_ will cause a "JavaScript output is disabled in JupyterLab" message in JupyterLab: somehow we'd have to use the MIME exclusively for JupyterLab and the display exclusively for Jupyter Notebook, but there doesn't even seem to be a way to test for which environment we're in.