Add support for other types of constrained figures

[spencerkclark]

Currently only width is enabled as a constraint. While for me this has been the most common use case, there are at least two other conceivable use-cases:

• A height-constrained figure with a fixed aspect ratio (and flexible width)
• A width and height constrained figure, with a flexible aspect ratio.

Internally I think it makes sense to create separate classes for these other types of constraints; we'll have to think a little more carefully how we might want to expose them in the interface. It could be something as simple as which keyword arguments one specifies (out of width, height, and aspect).