Add markdown plugins

[espenmn]

The markdown transform just loads one plugin: fenced code.

I tested some others and they all worked
.markdown(orig, extensions=['markdown.extensions.fenced_code', 'tables', 'extra', 'etcetc' ])

Is there a reason for not enabling all of them.

[pabo3000]

Especially the lack of nl2br is an annoyance. It should be configurable at least in the ZMI. I did it, but in an existing portal you have to delete and add again the transform markdown_to_html.

#18