Beta release of v3.0.0-beta.18

Changes:

• Add support for "tooltip" encoding channel.
• Add custom tooltip handler support (View.tooltipHandler).
• Add support for axis and legend titles to use signal values.
• Add nulls-histogram and nulls-scatter-plot example specs to illustrate strategies for incorporating missing values into charts.
• Fix event preventDefault bug (resolves error in Firefox).
• Fix autosize calculation: limit one re-run per propagation cycle.
• Fix ViewLayout to enqueue changed axis/legend groups for render.