idea: remove and distill data from/in charts

[ijlyttle]

I think there is an opportunity to provide some helper functions. I think we could use some purrr magic to provide functions like:

• remove_data(), which would go through a chart and, well, remove the data.
• distill_data(), which would go through a compound chart, see if there are repeated data, and if so remove them in favor of a data at the "top" of the object.

[ijlyttle]

Just to note that @AliciaSchep rightly suggests add_data() to add data to the "top" of an object.

She also notes that distill_data() could be done automatically as a part of the vegalite() function, perhaps with an argument not to do it.

As a followup, I wonder if this might also be done automatically as a part of our concatenation functions (|, &, +). If someone wanted to suppress that, perhaps we could offer an option like altair.infix_concatenation.distill.

[ijlyttle]

This is being done in the Python Altair package (vega/altair#951); for which we are thankful.