feature: optional git hooks

[ChristianMurphy]

closes #634, resolves #535, and fixes #633

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Acknowledges in CONTRIBUTING.md that we're trying to do Conventional Commits.

Adds hints on how to locally lint commit messages and even on how to automate this.

Funny anecdote: The hook caught problems with my initial commit message attempt for both all three of these commits. Writing in Sentence case with a full stop. it's an ingrained habit.

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This adds and documents a add-hooks script in package.json that installs husky for those who want git hooks but keeps hooks off by default.

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• This Contribution is under the terms of Individual Contributor License Agreements (and also Corporate Contributor License Agreements to the extent applicable) appearing in the Apereo CLA roster.