feat(axis): Intent to ship axis.tooltip

[netil]

Issue

#3603

Details

Implement realtime coordinate axis tooltip

axis: {
    tooltip: true, // default background color is

    // set backgound color for axis tooltip texts
    tooltip: {
         backgroundColor: "red",

         // set differenct backround colors per axes
         // NOTE: In this case, only specified axes tooltip will appear.
         backgroundColor: {
              x: "green",
              y: "yellow",
              y2: "red"
         }
    }
}

[coveralls]

coverage: 93.775% (-0.1%) from 93.906%
when pulling ec0b2b5 on netil:axis#3603
into 0dbb63b on naver:master.