nh3

[jochenklar]

This PR adds the nh3 to clean markdown content. The feature can be controlled by settings.MARKDOWN_CLEAN = True | False and for more fine grained control:

MARKDOWN_CLEAN = True
MARKDOWN_CLEAN_KWARGS = {
    'attributes': {'*': {'style'}},
    'filter_style_properties': {'color'}
}

to allow, e.g., style arguments, but only the color property.