Add responsive height and width option

[MarcSkovMadsen]

When creating data apps as in #217 you most often want responsive plots. I.e. plots that resize to the available width and height of the parent container.

With Vega/ Altair you can set the height and width to "container". I don't see something similar documented for Mosaic? See https://vega.github.io/vega-lite/docs/size.html#specifying-responsive-width-and-height

Could you please support or document? Thanks.

[derekperkins]

@tannerlinsley I know you've done a lot of work in this area