New feature c3.tooltip.show function

Change request: It would be very useful to have a

`c3.tooltip.show(data) `

method as a parallel to the position version

`chart.tooltip.position()`

c3.tooltip.show(data) would return True to show the tooltip and False would indicate the tooltip should not show.

The argument 'data' would be the same object as passed in as param-1 to the tooltip.position() method.