Add package.json

Please add a package.json to this repository so it can be added by projects using Cordova 7+. Thanks!

[Information on generating a package.json](https://cordova.apache.org/docs/en/latest/guide/hybrid/plugins/#publishing-plugins)

[Cordova 7.0 release notes](https://cordova.apache.org/news/2017/05/04/cordova-7.html)