Use git version of markdown for now

The latest version on haxelib is missing a patch for support for `c++`
as a language identifier in code blocks.

Author: tobil4sk

CodeCookBook.hxml

@@ -3,7 +3,7 @@
 
 --next
 
--lib markdown
+-lib markdown:git:https://github.com/HaxeFoundation/haxe-markdown.git
 -lib hxtemplo:git:https://github.com/Simn/hxtemplo.git
 -lib hxparse
 -cp src