Allow to pass a nesting_level option for HTML_TOC

Hello,

This pull request allows 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. For example:

 ~~~ruby
Redcarpet::Markdown.new(Redcarpet::Render::HTML_TOC.new(nesting_level: 2))
 ~~~

Resolve #267

Moreover, I've added some tests since there weren't any test related to this render. Please tell me if I'm doing it the right way ; I'm not sure about the assertions.

Have a nice day.