Update doc of individual datasets, expose to users

[garborg]

I feel like doc access from the repl, IJulia, and Juno would make the package much more usable. Format-wise, adding HTML just takes a couple extra couple lines in an existing R script -- not sure how much extra work rst, etc., would be.

Thoughts on format/API?

[asinghvi17]

We could overload Docs.getdoc?

[asinghvi17]

Another thing we could do, though it might take some manual effort, is to create a subclass of DataFrame called RDataset, which would behave exactly like a DataFrame - copy could even return a DataFrame - except for having some metadata about columns in a field. We could then use Docs.getdoc to display the metadata...