Refactor the commonmark converter class

Addresses #26.
I think this needs to reference $htmlRenderer instead.

https://scrutinizer-ci.com/g/thephpleague/commonmark/inspections/d9a558a8-aac1-47f3-a6bd-dac1dec28e71/issues/files/src/CommonMarkConverter.php?status=new&orderField=path&order=asc