Add hover interactivity to Plot.Line and Plot.Area.

Close #848.
I'll need to read through this more but one thing I am concerned about is that right now our assumptions for our projectors is that our projectors are either invalid or correct.  In the invalid case, xProjector(d, i) can return undefined (or a string or anything really).  Does your code run after the projectors are set up and if not, do you special case around these cases?