feat(event): add option for step-before/step-after charts for tooltip to match step behavior

[ebencollins]

Issue

#2287

Details

Optionally makes tooltip show for point of moused over step for step-after and step-before charts.
Fixes the previous inconsistencies when mousing between step transitions and fixes it for timeseries charts.
Added note to the documentation that data․type must be step, and updated the demo to reflect this.

It's worth noting that prior to 5d3a5ed, this behavior was default for charts with step-after that weren't timeseries, but not step-before. I'm not sure what was intended, so I added it as an option with default false to be safe, though I think it makes the step chart feel far more intuitive.

step-before example: https://i.imgur.com/mXywDti.png
step-after example: https://i.imgur.com/IBoR69K.png

step-after with tooltipMatch=false (also current behavior): https://i.imgur.com/4B9rx2d.png

[coveralls]

Coverage increased (+0.02%) to 89.968% when pulling 37941b8 on ebencollins:step-tooltip-placement into 1ce7333 on naver:master.

[netil]

@ebencollins thanks for the contribution!

I did some updates from your PR.
And the following is working example of the new tooltipMatch option.

line: {
    step: {
        type: "step-after",
        tooltipMatch: false
    }
}

line: {
    step: {
        type: "step-after",
        tooltipMatch: true
    }
},