snappy, shared, and programmatically triggered tooltips

[williaster]

This PR adds new functionality around tooltip behavior and customization 🎉

🏆 Enhancements
[xy-chart]

• Ability to "snap" the tooltip to the x or y value of a datum, by setting snapTooltipToDataX and/or snapTooltipToDataY. fixes add support for snapping tooltips to values #77
• Support for using the chart container for mouse events, instead of series or a voronoi. this is now set with the eventTrigger prop as 'series' [default], 'voronoi', or 'container'.
• The addition of container events necessitates shared tooltips, i.e., tooltips that contain data for for all series for the hovered x value. fixes [xy-chart][tooltip] support finding closest data points without voronoi #78
• Ability to programmatically trigger events using the eventTriggerRefs callback (see updated <LineSeriesExample /> for an example)
• adds innerRef prop which is set on the inner svg

[shared]

• the signature of onMouseMove in <WithTooltip /> now accepts an optional coords object of the shape { x: Number, y: Number }. If either or both of x or y is specified they will be used to set the the tooltips left and top instead of the event's coordinates.

[forms]

• adds active prop to <Button />

💔 Breaking Changes

• [xy-chart] the <XYChart /> useVoronoi prop is removed. instead use eventTrigger='voronoi

📜 Documentation

• [xy-chart] documents the above enhancements

🏠 Internal
[xy-chart]

• moves <XYChart /> static method to their own utils files
• breaks out several functions in chartUtils into their own files
• adds and uses sharedSeriesProps

TODO

• refactor shared tooltip series keys to use <*Series />-level seriesKey prop instead of label
• update readme
• add tests

[coveralls]

Coverage decreased (-3.7%) to 80.736% when pulling 8b02ffe on chris--customize-tooltip-behavior into 4bb987d on master.

[coveralls]

Coverage decreased (-3.7%) to 80.736% when pulling 8b02ffe on chris--customize-tooltip-behavior into 4bb987d on master.

[coveralls]

Coverage decreased (-3.7%) to 80.736% when pulling 8b02ffe on chris--customize-tooltip-behavior into 4bb987d on master.

[coveralls]

Coverage decreased (-3.6%) to 80.788% when pulling cf0245c on chris--customize-tooltip-behavior into 4bb987d on master.

[coveralls]

Coverage decreased (-1.8%) to 82.619% when pulling 0438f97 on chris--customize-tooltip-behavior into 4bb987d on master.

[coveralls]

Coverage decreased (-1.8%) to 82.619% when pulling 0438f97 on chris--customize-tooltip-behavior into 4bb987d on master.

[coveralls]

Coverage decreased (-1.8%) to 82.619% when pulling 0438f97 on chris--customize-tooltip-behavior into 4bb987d on master.

[coveralls]

Coverage decreased (-1.9%) to 82.527% when pulling 0438f97 on chris--customize-tooltip-behavior into 4bb987d on master.

[coveralls]

Coverage decreased (-1.9%) to 82.527% when pulling 0438f97 on chris--customize-tooltip-behavior into 4bb987d on master.

[coveralls]

Coverage decreased (-1.9%) to 82.527% when pulling 0438f97 on chris--customize-tooltip-behavior into 4bb987d on master.

[coveralls]

Coverage decreased (-2.01%) to 82.395% when pulling cdcb43f on chris--customize-tooltip-behavior into 4bb987d on master.

[coveralls]

Coverage decreased (-1.05%) to 83.355% when pulling 6828e96 on chris--customize-tooltip-behavior into 4bb987d on master.

[coveralls]

Coverage increased (+0.01%) to 84.419% when pulling 4e3f04e on chris--customize-tooltip-behavior into 4bb987d on master.