Implement automatic resize

[jimbojw]

Currently, the Scene's resize() method offers the API user a way to explicitly request Megaplot to recompute the offset and scale values based on a changed canvas size.

In addition, it would helpful to users to perform this resizing automatically. It should be possible to disable automatic resize in the event the API user does not want it.

Design question: What parameters can/should the user provide to inform how the resizing ought to be carried out?

See also: #16