v0.4.0: Added support for non-python notebooks and binder

This release adds support for non-python notebooks and the possibility to include links to binderhub-services (e.g. https://mybinder.org) in the docs.

Added

• Thanks to the efforts of @effigies in #3, #4, #5, #6, #7, #8, and #9 , we now support README.md files and non-python notebooks (see the bash example in the docs)
• sphinx-nbexamples now supports including a link to binder services with buttons like in the converted notebook. See the docs on including a link to binder

Changed

• Bokeh has been marked as not working in the conversion of notebooks (see #10)
• a bug with the thumbnails in the linkgalleries directive for sphinx>1.8.5 has been resolved (see
  cc402b2)