Feature: configurable list of showdown-extensions

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 ***--showdown*** (hence also a `.groc.json`-field) 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