feat(tooltip): Provide y position for tooltip position callback

[netil]

Issue

#3434

Details

Enhance providing y axis position value or function for tooltip.position callback argument

tooltip: {
     position: function(data, width, height, element, pos) {

         // pos: {
         //   x: Current mouse event x position,
         //   y: Current mouse event y position,
         //   xAxis: Current x Axis position (the value is given for axis based chart type only)
         //   yAxis: Current y Axis position value or function(the value is given for axis based chart type only)
         // }

         // yAxis will work differently per data lenghts
         // - a) Single data: `yAxis` will return `number` value
         // - b) Multiple data: `yAxis` will return a function with property value

         // a) Single data:
         // Get y coordinate
         pos.yAxis; // y axis coordinate value of current data point

         // b) Multiple data:
         // Get y coordinate of value 500, where 'data1' scales(y or y2).
         // When 'data.axes' option is used, data can bound to different axes.
         // - when "data.axes={data1: 'y'}", wil return y value from y axis scale.
         // - when "data.axes={data1: 'y2'}", wil return y value from y2 axis scale.
         pos.yAxis(500, "data1"); // will return y coordinate value of data1

         pos.yAxis(500); // get y coordinate with value of 500, using y axis scale
         pos.yAxis(500, null, "y2"); // get y coordinate with value of 500, using y2 axis scale

         return {
           top: 0,
           left: 0
         }
     }
}

[coveralls]

coverage: 94.326% (-0.01%) from 94.337% when pulling dc472bf on netil:tooltipPos#3434 into 19a6e1c on naver:master.