Add provider for flake8rules.com

[rpdelaney]

Pull Request Template

Description

Add provider for flake8rules.com.

Checklist

• All files have been linted with gofmt.
• Documentation has been updated.
• All tests pass.

[rpdelaney]

For this one I added strings.ToUpper() since the first letter of a rule id is always capitalized, so e.g. if the user gives f401 we should use the string that will actually find something, in my opinion. I won't die on this hill if you want that out though, just let me know. :)