🐍 Add parser support for additional markup features

[fwkoch]

This PR includes some work from SciPy sprints to support a wider variety of markdown syntaxes:

• roles from shortcodes, e.g. {{< role value >}}
• labels at the end of headings/images/etc, e.g. {#my-label}
• classes on spans, e.g. [content]{.python}
• colon fences can be used to create divs with classes