ENH: support vega-themes

[jakevdp]

This PR adds support for vega themes. For example:

import altair as alt
alt.themes.enable('dark')

from vega_datasets import data
cars = data.cars()

alt.Chart(cars).mark_point().encode(
    x='Horsepower',
    y='Miles_per_Gallon',
    color='Origin',
).interactive()

This requires your frontend to use the most recent release of vega-embed.

Fixes #781

[domoritz]

🎉 Thank you @jakevdp for adding this.