Add repository field to package.json

If we add the `repository` field in the form of `<repo_author>/<repo_name>`, it automatically fills up `homepage` and `bugs` field. See [the package.json](https://github.com/sindresorhus/pageres/blob/master/package.json) and [the repository page](https://www.npmjs.org/package/pageres) of [pageres](https://github.com/sindresorhus/pageres) as an example.