Update to Vega-Lite 3.2 and Vega 5.3 & support all new features. See
https://github.com/vega/vega-lite/releases/tag/v3.0.0 for Vega-Lite
feature lists.

Highlights

• new compound marks: mark_boxplot(), mark_errorband(), mark_errorbar()
• new transforms: transform_impute(), transform_joinaggregate(), transform_flatten() transform_fold(), transform_sample(), transform_stack()
• new facet encoding that is similar to the row and column encoding, but allows for wrapped facets
• new alt.concat() function that is similar to alt.hconcat and alt.vconcat, but allows for more general wrapped concatenation
• new columns keyword that allows wrapped faceting, repeating, and concatenation.
• many, many bug fixes
• tooltips can now be automatically populated using the tooltip mark configuration.
• ability to specify initial conditions for selections