Refactor HTML output methods

[jakevdp]

• adds alt.Chart.to_html() method
• adds Kaggle renderer, and refactors Colab renderer
• adds ability to specify whether html output is full or partial, as well as whether or not it uses require.js
• adds jinja2 to the core requirements for better HTML templating

[jakevdp]

I checked this branch on colab and confirmed it works properly... still need to check on kaggle.

[jakevdp]

I also checked this branch on kaggle and confirmed that it works correctly.