Upgrade html-pipeline and commonmarker

[paulfri]

Is your feature request related to a problem? Please describe.
This gem depends on outdated versions of html-pipeline and commonmarker. While the former is niche, the latter is broadly applicable and has substantial missing functionality given the upgrade from cmark to comrade.

Describe the solution you'd like
Upgrade these dependencies to new major versions.

Describe alternatives you've considered
We could use other markdown gems in our application.

Additional context
This probably has to be a major version upgrade, given that the pipeline config has changed substantially and is passed through via this gem.

[brettchalupa]

Thanks for opening this issue @paulfri! I think this would be good to do for the next major 6.0.0 release. 👍