Improve CI behaviours

[christophebedard]

This does a number of improvements:

• Detect normal GitLab MRs and external (GitHub) MRs
• Use base revision for CircleCI if provided (through the CI config)
• Detect PRs and base branch for Azure Pipelines
• Detect PRs and base branch for AppVeyor

And:

• Add "CI support" table to README describing each CI's behaviour and quirks
• Cleanup classes, remove unnecessary duplication

Closes #39

[codecov[bot]]

Codecov Report

Merging #65 into master will decrease coverage by 5.27%. The diff coverage is 24.21%.

@@            Coverage Diff             @@
##           master      #65      +/-   ##
==========================================
- Coverage   59.76%   54.48%   -5.28%     
==========================================
  Files           2        2              
  Lines         420      457      +37     
  Branches       58       72      +14     
==========================================
- Hits          251      249       -2     
- Misses        156      197      +41     
+ Partials       13       11       -2     

Impacted Files	Coverage Δ
dco_check/dco_check.py	54.28% <24.21%> (-5.29%)	:arrow_down:

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