Allow PNG-only mode

[jakevdp]

Because many python-generated vega plots result in data being embedded into the notebook, notebook size can quickly grow beyond what the browser can handle (see vega/altair#249). Would there be a way to optionally enable PNG-only mode, where on save (or perhaps on render) the live output is replaced by the PNG and removed? I think the JS could be tweaked to do this, but it's not clear to me how this could be easily enabled/disabled from the Python side.

cc/ @ellisonbg