Add basic assertions for the HTML_TOC render

Since we don't have any tests for the render object, we should ensure
that a minimum set of assertions are passing before trying to improve it
with additional stuff.
Allow to set a maximum rendering level for HTML_TOC

Allow the user to pass a `max_level` option when instantiating a new
HTML_TOC render object in order to limit the nesting level in the
generated table of content.
Merge pull request #279 from vmg/html-toc-render

Allow to pass a nesting_level option for HTML_TOC