Explain how to pass arguments to scripts

This is a small addendum to https://www.npmjs.org/doc/cli/npm-run-script.html that elaborates on how to pass arguments to scripts.

Sources:

- https://github.com/npm/npm/pull/5518
- http://unix.stackexchange.com/questions/147143/when-and-how-was-the-double-dash-introduced-as-an-end-of-options-delimiter