Closes #786.

This fixes #755

Add a config to set the footnote separator. Also remove rel/rev as they aren't really compatible with HTML5 and we already have classes set for refs and backrefs.

Fixes 723.

New `toc_tokens` attribute on Markdown class.
Contains the raw tokens used to build the Table of Contents. Users can 
use this to build their own custom Table of Contents rather than needing 
to parse the HTML available on the `toc` attribute of the Markdown 
class.

And other docs cleanup.

Additional CSS classes names can be appended to the admonition name
using spaces as separators.

The following markdown:

    !!! note floatright
        This is a floating note.

Generates the following HTML code:

    <div class="admonition note floatright">
        <p class="admonition-title">Note</p>
        <p>This is a floating note.</p>
    </div>