Add data_from_url option for Smart Cell Chart

[joeybellerose]

It would be awesome to have an option for the Smart Cell Chart to provide an option to use VegaLites data_from_url. This way data doesn't have to be only local. We could chart external data sources.

For example here's a huge list that the R community maintains.
It would be great to chart these without having to download them. 👍

[josevalim]

Maybe we can detect if a variable is a string and, if so, use it as a URL? A pull request along those lines would be welcome!

[josevalim]

We are not planning to do this for now because we do many things related to data inference. Perhaps the best step is to load this data into an Explorer dataframe first!