Better RTD implementation of examples/documentation #32

Currently, contributors cannot add examples. I generate examples from a big ipython notebook and convert them to RST with a shell script that calls nbconvert and does some regex magic to repair RST-style Sphinx module links embedded in the Markdown notebook cells (e.g. by default, nbconvert turns `~proplot.subplots.Figure` into ``~proplot.subplots.Figure``). This workflow also requires that every cell has already been executed, but committing cells with executed output is bad practice IMO -- ProPlot currently does not track or push cell output, thanks to nbstripout commands I've added to .git/info/attributes (see nbstripout for details).

In the near future, I need to commit the example notebook, use nbsphinx to automatically run each cell instead of running the cells manually myself, and configure nbsphinx with a formal nbconvert template/filter that repairs the links.

Alternatively, I may fork nbsphinx and implement this manually. Then I could add both my fork of sphinx-automodapi and nbsphinx as submodules inside proplot.