Fixes #279

Instead of adding a new CLI switch and trying to figure out whether CLI vs module usage is distinguishable, I have decided to create a sister extra for toc and make if behave the same way, but insert the TOC HTML to the MarkDown HTML (after the first heading) as well as preserving it as a property. This allows CLI usages the flexibility to just replace `toc` with `inline-toc` and get a table of contents right after the first heading without any worries about backwards compatibility or pollution the CLI switches. I believe this to be the cleanest solution.

…n as a result

`toc_html = property(calculate_toc_html(self._toc))` doesn't seem to work because `self` is not valid in that context so I had to hack around it with the useless proxy method.