Add in Interaction.BarHover

This interaction takes in a BarPlot and calls callbacks when the user
mouses over a bar or mouses out of a bar.

Close #761.
Which direction is "parallel to the bar"? I feel the phrasing "extending the same direction as the bar" would be easier to understand.

You should also mention that it defaults to `"point"`.

These docstrings should be combined into one, and the setter declaration should go first.