Move the contents of qri render into a new subpackage for html rendering

[b5]

@dustmop had a point about testing viz template execution in #137

I'm now wondering if we should move a bunch of qri render from lib into a new subpackage in dataset. Starting with this code, the only packages in use are part of of the dataset package family:

https://github.com/qri-io/qri/blob/abb36a425321576fe922ab72bdf2b59e8d212ce4/lib/render.go#L102-L175

It'd be great to move this out of the main qri codebase, as I'm hoping we can add some nicer helper functions to the templating environment.

[b5]

we did this! new package is github.com/qri-io/dataset/dsviz