Avoid HTML escaping for table-of-contents' titles

Hello,

By default, the `HTML_TOC` render object used to HTML escape titles (see vmg/sundown#90) but in order to be consistent with other render objects, let's avoid this escaping and provide an `:escape_html` option.

Thus, the default behavior is more expected but it's still possible to safely display titles.

Fixes #255.

Have a nice day.