Support top-level padding / autosize property

[peller]

Support the padding property, in particular, the ability to use "strict" as a value such that width and height become constant. This is essential in order to do page layout.

See #1214

[kanitw]

Related Note for Vega 3

Vega 3 adds a new top-level "autosize" property to set the layout mode. The allowed options are "pad" (the default, similar to "padding": "auto" in Vega 2, the chart dimensions will be expanded as needed to fit the chart, axes and legends), "fit" (similar to "padding": "strict" in Vega 2, the data rectangle size will be decreased in an attempt to force the chart to fit within the given width and height) and "none" (no resizing is performed, and the chart size is solely determined by the width, height and padding settings).

[kanitw]

Will be done as a part of #1778