Parsedown module?

[sbrl]

Perhaps we should add a parsedown module that adds Parsedown as a parser to Pepperminty Wiki. It should be optional and not included by default since this module might add some significant weight to an installation.

• Add support for extensions that aren't included in the build by default (issue Optional modules that are not included by default #10)
• Create parsedown extension

[sbrl]

Done! We now have a parsedown module extension that's actually worth using.