Vega client plugin

@zachmullen - just got this working.  The idea is to upload a Vega spec somewhere, then edit its metadata to have a "vega" attribute, set to a truthy value (I've just been using the string "true").  This will cause the item view to render the Vega visualization contained therein.

There are obvious improvements to be made, like disabling the rendering if the attribute reads "false", and not loading the file if it's too big, etc. but this demonstrates the core idea.

You can get sample Vega spec files here: http://trifacta.github.io/vega/editor/ but I also have a sample one that is working, so I can just send that one to you if you like.