Add programmatic export utility based on selenium and chrome-headless

[jakevdp]

This allows you to save altair charts to PNG without going to the browser.

Dependencies can be installed this way:

$ conda install selenium PIL
$ conda install -c clinicalgraphics chromedriver

Then you can use the utility this way:

import alt
chart = alt.Chart(...)  # make your favorite chart here

from altair.utils import save_spec
save_spec(chart.to_dict(), 'mychart.png')

Eventually we can make this more streamlined for the user (e.g. chart.save('mychart.png')) but we'll have to figure out the dependency story... and I wanted to get the initial version of this utility committed so that I can use it to build docs for the release candidate.

[jakevdp]

This clearly could use some improvements, but I'm going to merge so that I can incorporate it into #552 and get the doc build working.