Make nodejs_npm_global_packages variable easier to use

[Jeff Geerling]

Basically, see @oxyc's comment here: geerlingguy/ansible-role-homebrew#33 (comment)

We could allow list items to just be string, and that would be the default filled in for a name parameter if name isn't provided. That way if someone sets up the packages like:

nodejs_npm_global_packages:
  # Install a specific version of a package.
  - name: jslint
    version: 0.9.3
  # Install the latest stable release of a package.
  - node-sass

...it would be perfectly valid!