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

Users can use `Interaction.Hover` with `Plot.Line` and `Plot.Area`. The `onHoverOver` callback will be passed the closest point to the cursor by X (Y breaks ties) within 15 pixels.

Close #848.
>> What happens if we call render multiple times? Seems to append circles onto the foregroundContainer while switching references

Creation of the fake target occurs in `_setup()`.