Environment file download alongside .ipynb and .pdf

[jmason86]

In the book that @mbobra and I co-edit (helioml.org), we have contributed chapter from various authors and each can have different and even conflicting environments. This means that we can't have a single environment file for the whole book, we need one for each chapter.

A solution that seems like it would be really consistent with the existing implementation is to have the .yml appear in the rendered notebook when you scroll over the download icon (which already pops up the .ipynb and .pdf).

We could include a link to the .yml in each notebook itself. That file could be hosted inside our repo or elsewhere. This doesn't seem as elegant as our desired solution though.