refactor(vega): Move to Vega 3.0, VegaLite 2.0

[jeffbaumes]

• Remove JSON templating that was used for Vega 2.x specs.
• Make a new VegaView superclass that utilizes a generateSpec method instead of a JSON spec.
• Remove BulletChart, Heatmap which had limited utility.
• SurvivalPlot is taken out of the build for now until we decide what to do with it.
• [x] Figure out what to do with SurvivalPlot (remove for now).
• [x] Fix tests.
• [x] Fix examples.
• [x] Fix dynamic line chart
• [x] Fix resize examples.
• [x] Update documentation.

[codecov-io]

Codecov Report

Merging #509 into master will decrease coverage by 7.24%. The diff coverage is 43.83%.

@@            Coverage Diff             @@
##           master     #509      +/-   ##
==========================================
- Coverage   47.72%   40.47%   -7.25%     
==========================================
  Files          62       58       -4     
  Lines        1645     1465     -180     
==========================================
- Hits          785      593     -192     
- Misses        860      872      +12

Impacted Files	Coverage Δ
plugins/vega/index.js	100% <ø> (ø)	:arrow_up:
plugins/vega/BarChart/index.js	92.85% <100%> (+17.85%)	:arrow_up:
plugins/mixin/index.js	100% <100%> (ø)	:arrow_up:
plugins/vega/ScatterPlot/index.js	18.18% <11.11%> (-56.82%)	:arrow_down:
plugins/vega/ScatterPlotMatrix/index.js	20% <12.5%> (-55%)	:arrow_down:
plugins/vega/GanttChart/index.js	40% <33.33%> (-35%)	:arrow_down:
plugins/vega/BoxPlot/index.js	9.52% <5.26%> (-65.48%)	:arrow_down:
plugins/vega/LineChart/index.js	11.11% <6.25%> (-19.66%)	:arrow_down:
plugins/mixin/VegaView.js	69.23% <69.23%> (ø)
plugins/vega/Histogram/index.js	13.33% <7.69%> (-61.67%)	:arrow_down:
... and 1 more

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[jeffbaumes]

@ronichoudhury ready for review.

[waxlamp]

There are still a few "regressions" (not reflected in the tests) caused by this PR, but I think we should merge this now and fix the issues in future topics. Here's a list:

• 511: shapes don't appear in scatter example (but do in scatter-solid)

• 512: the axis ranges do not update naturally in the line-dynamic example

• 513: the line-points example doesn't work anymore, because the new LineChart doesn't listen for click events like the old one did

[waxlamp]

@jeffbaumes since you're away, I'll press the button. Hope you don't mind :)