nl2br and attr_list compatability.

Fixes #177. When using both extensions, breaks (`<br>`) must have a linebreak (`\n`) after them before attr_list is run. This patch reorders the treeprocessors so that happens ('attr_list' runs after 'prettify' not before).

Also had to alter headerid extension so it runs after 'prettify' or it would run before 'attr_list' if loaded before 'attr_list' by user.