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>

If you have existing documents that use the legacy attributes format,
then you should enable the legacy_attrs extension for those documents.
Everyone is encouraged to use the attr_list extension going forward.
Closes #643. Work adapted from 0005d7a of the md3 branch.

We started with the numbers before HTML5 was a thing and we thought
there might be an XHTML2. Today, we know that all we have are HTML style
tags and XHTML style tags. Nothing else really matters in the real
world.

Note that if '(x)html1' '(x)html4' or '(x)html5' are passed in, the
number is stripped/ignored. Users shouldn't need to change their code
for this.