Add new method to access check run API

[pllim]

Fix #34

No idea how to test this. And even if this add_check works, need to discuss how to actually use it, as it provides a lot more options than status checks.

TODO

• Discuss how to use this.
• Update blueprints to use this. @astrofrog will do this part.
• Add tests. Cannot find a good way to test just the new method with mock and still have the test be useful.
• Add change log.

NOTES

• App owner needs to give app itself checks read/write access.
• Repo owner where the app is installed needs to update app installation permission as well.

PROOF OF CONCEPT

pllim/playpen#19 (Ops, the status check area disappeared when I closed the PR. But note that the checks also appear as statuses at the bottom of the PR like a regular status. Like Tom R said, checks are like fancy statuses.)

[codecov-io]

Codecov Report

Merging #45 into master will decrease coverage by 1.5%.
The diff coverage is 6.66%.

@@            Coverage Diff             @@
##           master      #45      +/-   ##
==========================================
- Coverage   85.93%   84.43%   -1.51%     
==========================================
  Files          14       14              
  Lines         775      790      +15     
==========================================
+ Hits          666      667       +1     
- Misses        109      123      +14

Impacted Files	Coverage Δ
baldrick/github/github_api.py	71.29% <6.66%> (-3.14%)	⬇️

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[pllim]

Other than foregoing the test, I think this is ready to go, @astrofrog . Perhaps this will be tested indirectly when you test a blueprint that uses this method?

[astrofrog]

Minor request but looks good otherwise!