Improve Keyword detector accuracy for .cls and .yaml files

[KevinHock]

🔭 [Keyword Plugin] Add cls and yaml special cases …
Require quotes in .cls files
Skip secrets starting with {{ and ending with }} in yaml files
🐍 [Keyword Plugin] Add test using keyword_exclude
🎓 [Usage] Minor nits …
Add period to end of all help strings
Formatting in usage_test.py

[KevinHock]

TODO: Make quotes required for .java files as well Done in fe3e989

TODO: Add to FALSE_POSITIVES:
- > for Ruby e.g. :some_password => params[some_thing]
- "dummy for e.g. "dummy value"
- :: (from an internal test case)
- null.*' (from an internal test case) Done in 2f21d3d