Improve integration with xvega

[marimeireles]

Thanks to @wangfenjin we now have full support to xvega, as long as we use a json instead of the xvega mini language.
The code to do so is here.
I don't expect the code to be too hard to adapt, since both kernels were built with the same purpose in mind.
Feel free to ask any questions here or in the gitter channel if you wanna tackle this issue!