A document option (e.g., for JSDOM)

[mbostock]

Rather than using the global document, let’s have a top-level document option that you can pass to Plot, and it will use that.

[Fil]

For more context: #847

[Fil]

Currently we mostly use d3-selection’s create method, and a few direct calls to document.createElement / createElementNS / createTextNode.

create is used most of the time for svg types, but in the color legend functions there are some div and canvas xhtml elements.

I'm not sure how to proceed. Should we pass document or a special create function to render? Or attach it to the plot object somehow?