Add CommonMarker markdown provider

[ojab]

Fixes #1157

Description

Add CommonMarker as a markdown provider. Slower than redcarpet, but has an actual standard.

ojab [ ~/src/yard ]$ time bundle exec ./bin/yard --markup markdown --markup-provider redcarpet
…
real	0m15,173s
user	0m13,997s
sys	0m0,578s
ojab [ ~/src/yard ]$ time bundle exec ./bin/yard --markup markdown --markup-provider commonmarker
…
real	0m16,851s
user	0m15,851s
sys	0m0,493s

OTOH on some big work project difference is insignificant:

$ time bundle exec yard  --markup markdown --markup-provider redcarpet
…
real	10m0,707s
user	9m17,975s
sys	0m17,672s
$ time bundle exec yard --markup markdown --markup-provider commonmarker
…
real	10m6,785s
user	9m25,962s
sys	0m17,463s

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).

[ojab]

Marking as ready for review because of First-time contributors need a maintainer to approve running workflows. Learn more..
bundler exec rake passes locally.

[coveralls]

Coverage increased (+0.005%) to 93.419% when pulling 7b7a7d3 on ojab:add_commonmarker_provider into cfa62ae on lsegal:main.

[ojab]

Ok, CI is green after restricting commonmarker to ruby >= 2.3.

[lsegal]

Great add, thanks!