Expose a way to add vega spec & not require it to be formatted like Cedar normally would expect

@tomwayson After playing around with some of those specs Andrew was talking about today, and looking over some of the samples he's put together it seems to me that we are overcomplicating some things. We have no way to just input a straight vega spec and allow vega to handle the data response. I think we should have the ability to just take the raw vega spec and not expect it to be in a template, have mappings, etc.

Thoughts?