Add a new option exclude_committers to ignore some committers

[Xiaokang2022]

You may need to clear the cache (the cache file in the folder .cache) after modifying this option for it to take effect.

Example:

# mkdocs.yml
plugins:
  - git-committers:
      exclude_committers:
        - web-flow

close: #76

[Xiaokang2022]

@ojacques PTAL, it's easy, thanks!