Add animated transitions to Vega.

[jheer]

Animated transition support is not planned for the initial 3.0 release, but would be nice to add in a subsequent minor release (e.g., 3.1). This issue is a migration of uwdata/vega#94.

Possible steps for implementing animated transitions in Vega 3:

• [ ] Design Vega 3 animation syntax (allow per-item duration/ease/delay within encode sets?)
• [ ] Add interpolation and scheduling support to vega-scenegraph
• [ ] Update Scale transform in vega-encode to persist previous scales
• [ ] Update vega-parser to allow access to previous scales
• [ ] Update Encoder in vega-encode to snapshot scenegraph item values prior to animation
• [ ] Update vega-view to orchestrate animation and retain animated exiting items (as needed)