Run hook on specific directories and files

[klmcadams]

• Run the add-license-headers hook on all python files within directories named src, examples, and tests.
  • Also run the hook on all proto files.
• Updated README with more regex examples, and how to add files to the .pre-commit-config.yaml files.
• Included ["non_compliant"]["missing_licenses"]["MIT"] check in the function that finds files that are non-compliant.

Note: the update to check for missing_licenses requires the LICENSES/MIT.txt file to be in the root directory of the repository, otherwise it's not going to work correctly.

Closes #50

[RobPasMue]

Some minor changes - otherwise LGTM!