chore(changelog): add commit-msg hook to verify commit messages

This adds a Git commit-msg hook which runs after a commit message is written. It verifies that is matches a certain spec as defined by the AngularJS document [here](https://docs.google.com/document/d/1QrDFcIiPjSLDn3EL15IJygNPiHORgU1_OOAqWjiDU5Y/edit#). This will allow for automated generation of Changelog.md in the future.

Migration notes: Please run `ln -sf ../../validate-commit-msg.js .git/hooks/commit-msg` to install the commit hook.