Better argmax handling in DiscriminationThreshold

[falcaopetri]

Although documentation allows DiscriminationThreshold(model, argmax=None), current implementation raises an exception since it assumes argmax to be a string.

I have made the following changes:

1. Allow argmax to be None (as already documented)
2. Raise error in case argmax is an invalid metric (similar to the check of exclude)
3. Add tests to check argmax
4. Improve docs of argmax and exclude
5. Fix typo in CONTRIBUTING.md

CHECKLIST

• [x] Is the commit message formatted correctly?
• [x] Have you noted the new functionality/bugfix in the release notes of the next release?
• [ ] Included a sample plot to visually illustrate your changes?
• [x] Do all of your functions and methods have docstrings?
• [x] Have you added/updated unit tests where appropriate?
• [ ] Have you updated the baseline images if necessary?
• [x] Have you run the unit tests using pytest?
• [x] Is your code style correct (are you using PEP8, pyflakes)?
• [ ] Have you documented your new feature/functionality in the docs?

[codecov[bot]]

Codecov Report

Merging #1202 (8dd58ce) into develop (1852319) will increase coverage by 0.02%. The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           develop    #1202      +/-   ##
===========================================
+ Coverage    90.40%   90.43%   +0.02%     
===========================================
  Files           90       90              
  Lines         5077     5089      +12     
===========================================
+ Hits          4590     4602      +12     
  Misses         487      487              

Impacted Files	Coverage Δ
yellowbrick/classifier/threshold.py	98.71% <100.00%> (+0.10%)	:arrow_up:

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