Avoid HTML escaping for table-of-contents' titles

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.
Merge pull request #433 from vmg/html-toc-escaping

Avoid HTML escaping for table-of-contents' titles