add optional support for attr_list extension in mkdocs

[mike-scott]

This is limited to direct attribute support as it will be included in the tag directly: { align="right" class="something" } becomes:

You cannot use: {: .class} as this becomes: <img : .class />

Signed-off-by: Michael Scott [email protected]

[mike-scott]

Not sure if you will automatically run the test output in a PR?