Update extension for new markdown version

[Wainggan]

I'm going to be completely honest, I haven't a single clue as to what I'm doing, this is my first attempt at contributing to anything.
The extension didn't seem to work with the markdown library anymore, so here is my attempt to fix it. Feedback would be sincerely appreciated.

[paolini]

You pass 0 as priority. In the previous version priority was described as ">raw_html". I have checked that raw_html has priority 20. I wonder if 0 is correct or 30 should be used.

@Wenzil are you willing to update the package?

this would solve issue #7

[Wainggan]

I wasn't aware that ">raw_html" meant anything like that, so I chose 0 because it worked with my (albeit basic) tests. If raw_html originally resolved to 20, it would probably be a good idea to edit 0 to something that fits around that better.