Add an option for specifying the relative position of the tooltip with respect to the chart width or height

In some cases, on small screens, the default tooltip positioning is not optimal: part of the tooltip cannot be displayed on the canvas, see example below. The purpose of this task is to let the user define the positioning of the tooltip as a coefficient of the chart width or height. Note that ChartJS comes with "xAlign" and "yAlign" options for tooltips, but these apply to the tooltip caret relative position only.