Demo implementation of hover tooltips for Slate

Not ready for general release.

Currently tooltips only work on VerticalBar and HorizontalBar plots. To use them, project the desired tooltip text on to "tooltip-text":

barPlot.project("tooltip-text", "x"); // displays the x value as the tooltip
barPlot.project("tooltip-text", (d: any) => String(d.x).toUpperCase); // all caps