Add 'mistune' markdown parser as an option for markdown2html

[Mistune](https://github.com/lepture/mistune) is a pure Python markdown compiler, which compiles itself using Cython if you have Cython available at install time. It describes itself as 'inspired by marked', and the author mentions in a [blog post](http://lepture.com/en/2014/markdown-parsers-in-python) that his idea was to port marked to Python, so we should be able to expect highly consistent results.

Advantages:
- Customisability: we can subclass and override methods if we need to change behaviour. The readme has [a simple example](https://github.com/lepture/mistune#renderer) of integrating with pygments for syntax highlighting.
- Installation: It's just another Python package to install - it doesn't need compilers, or another runtime.
- Performance? It's probably not the most optimised implementation, but I guess there's a considerable saving from not starting up a separate process for every markdown cell. I haven't benchmarked, though.

At present, I've put this after node+marked and before pandoc in the order of preference. The tests pass when I comment out the marked version.
If we are adopting mistune instead of node, let's replace this whole `markdown2html` with just:

    markdown2html = markdown2html_mistune

below. We can make `mistune` a dependency of nbconvert.