Use mainstream mistune

[rossant]

cc @lepture @bollwyvl

As far as I remember the changes were relatively minor. I'll give more information when I find the time to delve into the code again. It was something along the lines of "having a non-renderer Markdown parser that allows one to write callback methods that don't render anything".

[lepture]

In the meanwhile, I am designing lepture/mistune#62

[rossant]

Feel free to have a look at the code in ipymd, that's a bit what I did (there's a BaseLexer etc)