Add key to visualizations exported

[learnspyral]

In Voyant when you go to Export a Visualization you get a graphic that doesn't have the key provided in the interactive panel. Can there be an option to include the key, both in Voyant and in Spyral?

[ajmacdonald]

This will be a challenge as the legend/key is typically rendered in HTML while the visualizations are rendered using SVG or Canvas.
Related discussion here: #12

[ajmacdonald]

Note to self: look into https://github.com/zumerlab/snapdom