Example gallery about altair

[ijlyttle]

I am with you here! In fact, @haleyjeppson has started working on this privately, so I take this opportunity to invite her to be less-private (sorry Haley)!

I had a short opportunity to look over Haley's shoulder a few days ago, and she is finding both confirmation and identification.

Perhaps we could start with her work when she is ready. As an aspiration, maybe we could help with prose.

There is the issue of how to import the vega datasets, I like the idea above of hopping onto reticulate. I'll open another issue on that.

[ijlyttle]

@haleyjeppson has contributed a monster set of examples from the Altair gallery, #59

I will begin the process of incorporating it into a gallery on the pkgdown site.

[AliciaSchep]

Woohoo! 🎉 🎉 🎉