Support prefixing autogenerated IDs

[borgar]

PHP Textile has a feature where it adds an autogenerated prefix to every autogenerated ID:

foo[1]

fn1. bar

Becomes:

<p>foo<sup class="footnote" id="fnrev15543656566092fe9dadd44-1"><a href="#fn15543656566092fe9dadd44-1">1</a></sup></p>

<p class="footnote" id="fn15543656566092fe9dadd44-1"><sup>1</sup> bar</p>

Whereas textile-js does:

<p>foo<sup class="footnote" id="fnr1"><a href="#fn1">1</a></sup></p>
<p class="footnote" id="fn1"><a href="#fnr1"><sup>1</sup></a> bar</p>

Preferably this should be an opt-in via an option.