Support built-in Markdown extensions

[petri]

The markdown renderer used in this package supports a number of extensions - that are already distributed with the package as well.

Thoughts on enabling (at least some of) them by default in websauna.blog? I'd like to see at least:

• codehilite
• abbr
• tables
• links
• footnotes

As an example, here's a draft links implementation.

For abbr, tables and footnotes extensions, I think just adding their name to the extensions list passed to markdown() is enough - no new code is needed.