Proposal: replace allow_contributor option with allow_non_author option

[ajlake]

Because allow_contributor includes the PR author and overrides the allow_author option, you are unable to achieve the following workflow:

Scenario: you want to enable an approver who sees a small typo in an otherwise good PR to fix it. However, this policy doesn't quite do what you want because allow_contributor overrides allow_author.

    options:
      allow_contributor: true
      allow_author: false

Proposal is to deprecate allow_contributor and instead have allow_non_author which functions the same but is mutually exclusive.