Change to ParsedownExtra

So the original PR (https://github.com/octobercms/library/pull/11) I mentioned that ParsedownExtra was not working properly. This was all to do with the singleton implementation of it. If you used the following you would get an instance of Parsedown and not ParsedownExtra:
```
ParsedownExtra::instance()->text($text)
```

The changes made in this PR does however bypass this bug and allows the use of Markdown Extra. Tested with slightly modified blog component and it works beautifully.