Use id instead of name for TableOfContentsFilter

Closes #135

I also added the [aria-hidden](http://www.w3.org/TR/wai-aria/states_and_properties#aria-hidden) attribute, which tells assistive devices that this element is not actually visible. We already set this on GitHub, so I thought it would make sense to be included upstream.

/cc @jch @josh 


Think so.

https://mathiasbynens.be/notes/html5-id-class
https://mathiasbynens.be/demo/crazy-class

Some good resources on `id` encoding.