made showdown-extension configurable via commandline and project-json

The showdown-extension “github” which replaces double-tidle-wrapped texts with a strike-through equivalent (like ~~this~~) does not work reliable - especially if one (like me) uses the tilde in pre-formatted texts.

Therefore I implemented a new commandline-switch (and project-json-field) ***--showdown*** to specify which showdown-extensions to load during the generation-process. The default value ***github,table*** is 100% compatible to the previously hardcoded list of showdown-extensions.

I'd be (very) glad if you pull in these changes.

Cheers + Thx,
Stephan