Add headless mode

[freeman-lab]

We should provide a headless mode that allows visualizations to be generated without a running server. The model is to embed data from the client directly into a template that renders the visualization by pulling an pre-bundled version of js/css, which we will ship with the client. The most common use case will probably be generating visualizations within Jupyter notebooks. We can begin by adding this functionality to the Python client and then expand to other clients.

A prototype is on the headless-experimental branch but that version still requires pulling the js/css from an already running server. As soon as lightning-viz/lightning#38 is finished we can package that with the client, which will also probably require some changes to the embedding / template.