Unit tests

[markgeraty]

Not necessarily testing how well it filters (let the js-xss library handle that), but test that it works as a showdown extension on both the client and server side.

[tivie]

I've written an article regarding XSS vulnerability. Might help creating some test cases.

[tivie]

https://github.com/showdownjs/showdown/wiki/Markdown's-XSS-Vulnerability-(and-how-to-mitigate-it)