Add license checker/crawler

[hbowden]

Add a npm package that checks the license of all our dependencies so that we only use permissive licenses such as MIT, BSD, ISC, etc.

[hbowden]

Could use something like this: https://www.npmjs.com/package/node-license-validator

Would be nice to add to the CI build process.

[stuller]

work started in PR #391