Don't parse markdown in code blocks

[seancdavis]

It would be awesome if code blocks would syntax highlight based on the language, but they should at least ignore markdown rules.

For example, if I have something like this:

[```]
my_var = another_var
[```]

then var = another would be italicized, which is undesirable.

[lepture]

GFM is not enabled by default. Working with the new codemirror at https://github.com/lepture/editor/tree/bower But I don't have much time on this project right now.