Include valid config attributes with plot schema

[cpsievert]

It would help me throw informative errors/warnings in the R package.

[etpinard]

Good call. I tried that once (in #1188), but was a little too ambitious with that PR.

[etpinard]

could be a great community contribution.

[jonmmease]

This would also be helpful for plotly.py. We've fallen behind the valid list of config options a couple of times now. e.g. plotly/plotly.py#1290