Exportable extension

Resolves #38. This pull request supersedes #159.
documentation comments: 
(I'm happy to implement these when I add code samples after this is merged, just wanted to include FYI...)

- We're going to need to remove the indent for lines 686-692. When we run the build-documentation script, the indent stays and this paragraph is formatted as code rather than paragraph in the resulting markdown. The content itself is great, thanks for including.

- Would it be ok with you if we move line 670-695 to line 24 (just above "var exportable = function  () ...")? This means that in the generated docs, the visualization that you need to add to your Contour instance (`.exportable()`) is listed first, before any of the functions that you can call on your instance once that visualization has been added. This is how we've done it in the other visualizations (like cartesian.js).