Use toc_tokens to generate the TOC

This patch improves the consistency of TOC levels, so now the level is always
equal to the N in the `<hN>` tag. It also allows users of the MkDocs theme to
set the navigation depth to show in the TOC panel (defaulting to 2).
Resolves #1910 and resolves #770.

setup.py

@@ -58,7 +58,7 @@ setup(
         'Jinja2>=2.10.1',
         'livereload>=2.5.1',
         'lunr[languages]>=0.5.2',
-        'Markdown>=3.0.1',
+        'Markdown>=3.2.1',
         'PyYAML>=3.10',
         'tornado>=5.0'
     ],