enable table support for CommonMarker

[notEthan]

Description

In switching from redcarpet to commonmarker, I found pipe-delimited tables did not render. commonmarker supports these and enabling them is a matter of merely adding :table to the extensions parameter of CommonMarker.render_html, and enabling existing tests for this markdown provider.

this was enabled by default for redcarpet long ago in #765 and I think it is good to do for commonmarker too.

Completed Tasks

• I have read the Contributing Guide.
• The pull request is complete (implemented / written).
• Git commits have been cleaned up (squash WIP / revert commits).
• I wrote tests and ran bundle exec rake locally (if code is attached to PR).
  (sort of, I just turned on existing tests)

[lsegal]

Thanks for this quick fix!