Add .save_html() method

I am using example from documenation:

html = chart.to_html()
with open('whatever.html', 'w') as f:
    f.write(html)

here is suggestion: wrap this code into save_html method, or accept to_html(file=) keyword for saving.

[stsievert]

We could provide save as an alias to that function if you think it would be more clear.

I think this a documentation issue, and I’ll try to address this with a PR sometime this week.

[jakevdp]

In v2, to_html is now gone, and save('chart.html') works.