Publish plugin - npm

It tries to resolve issue #202 by providing the following configuration in `.drone.yml`:

    publish:
      npm:
        username: foo
        email: foo@example.com
        password: bar
        registry: https://npm.example.com/me/ #optional
        folder: my-project/node-app/ #optional
        tag: 1.2.3 #optional

**Notice** that it uses npm trick to provide the `~/.npmrc` file instead of running command `$ npm adduser`, so it should be updated once npm supports setup credential without asking input from stdin or the trick is deprecated.