Use composer to bring in erusev/parsedown and erusev/parsedown-extra

[reedy]

Per better practices, you should use composer to bring in erusev/parsedown from https://packagist.org/packages/erusev/parsedown and erusev/parsedown-extra from https://packagist.org/packages/erusev/parsedown-extra

[reedy]

#19 does this and a lot more