Add ability to layer vega / vega-lite on top of eachother

[metasoarous]

It seems like it should be possible to pass vega and vega lite descriptions to the client, use the js lib to parse the vega-lite into vega, and then merge everything together, so that you could do nitty gritty tuning with vega, but take care of the trivial bits using vega-lite.

Ideally, we'd just find a way of translating vega-lite to vega on the server, but I'm not sure how best to do that short of using the socket to request translations from the js/browser, or call out to a js/node process. Perhaps we can load vega in nashorn?

Short of that, perhaps we just pass along both descriptions to the browser and have it merge them together. But this would provide much less flexibility that being able to just translate vega-lite to vega and build upon that description.

[zane]

Perhaps we can load vega in nashorn?

This appears to be an example of someone doing just that.

[metasoarous]

Very cool. Thanks @zane! This may be helpful for #36 as well.