npm state module: Add parameter to specify a custom NPM server

The [pip-state](http://docs.saltstack.com/en/latest/ref/states/all/salt.states.pip_state.html) state module offers the parameters `index_url` and `extra_index_url` to pass a custom PyPI server to Salt.

The `npm` state module is lacking such an option, therefore I cannot use my companies NPM server to manage private NodeJS packages.

I think a parameter to specify a custom NPM server would greatly improve the `npm` state module.