Add support for additional Admonition classes

[waylan]

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>

This is an update to #580 which has been rebased on master and conflicts resolved.