sketch of zoom approaches

[mbostock]

For #1590. Related #1738. This sketches out four possible strategies for implementing zooming. All are currently implemented as a render transform (to prototype). I need to expand the first strategy to re-run the mark initializer so that the axis mark can recompute the desired ticks. Or, we take the approach suggested in #1738 where marks have to know about zooming and handle incremental re-rendering in a special method…