Add support for detecting "unlicensed" modules

So far, a listed package license will be marked as UNLICENSED only if it has set the key private to true in package.json - but as (according to the npmjs documentation) one can't publish private packages to NPM registries, I now added support for detecting "license":"UNLICENSED" in package.json in order to allowing to publish unlicensed packages to NPM registries.

Full Changelog: v2.1.1...v2.2.0