~~~~~~~~~~~~~~~~~~~
- Added support for OpenGraph tags with colons (e.g. ``image:width``
and ``image:height``).
- Modified ``meta_tags`` (and by extension also ``meta_tags_html``) to
handle keyword arguments differently: Falsy values are discarded,
``None`` causes the complete removal of the tag from the dictionary
respectively from the HTML output.
- Separated the HTML rendering into ``format_meta_tags`` to make it
reusable.