Added option to calculate height and/or width on render.

By default, uses height and width of anchor DOM element.
Can optionally specify function to use for calculation.

Doesn't address all the auto-sizing features listed in issue #295, but I'm finding it very useful for these scenarios:
- Have chart fill parent div, thus controlling chart size via CSS
- Have chart dynamically resize in response to user interactions (e.g. user changes top N count with slider)