Initial code for pinch selection.

This is the foundation code to draw pinch lines between each pair of axes.
The actual filtering is not yet implemented.
Add an example to demonstrate pinch selection.
Add support for removing a pinch.

This is currently implemented by a single click in the area of the pinch.
Simplify code.

I discovered d3.mouse, so now the initialization is properly done on
dragstart. I also reuse the dimensionsForPoint function to get rid of some
partly code duplication. As a consequence, pinch is now behaving more
correctly when it is started outside the left or right bounds of the plot.
Separation of concern: don't clear in drawPinchLine.

This (imo) is slightly cleaner than what I had before.