Use shape to encode data

[domoritz]

Even though this is not supported by pandas scatterplots, I would love to encode data with shape as in addition to color, x, y, and size.

[jakevdp]

I'd be open to making this API a superset of pandas... particularly for the scatter function, which is the one that maps really well onto the grammar of vega-lite.

Maybe we should add a scatter argument for each of the available encodings?