Add 'mistune' markdown parser as an option for markdown2html
Add highlighting of GFM code blocks with mistune
Make mistune a requirement for nbconvert
Add deps for nbconvert to HTML for 'notebook' target

Also, nbconvert no longer requires Sphinx.
Integrate pass-through of mathematical expressions with markdown rendering
Version dependency on mistune
No need to install nodejs on Travis
Add support for \begin foo \end math blocks
markdown2html test doesn't rely on pandoc
Rename mistune renderer class
mistune rendering is the default
Fix code block rendering when lexer not known
Fix mathjax pass-through with mistune
Merge pull request #6028 from takluyver/markdown-mistune

Add 'mistune' markdown parser as an option for markdown2html