Allow options to npm install

Can be useful if you're wanting to install something in particular.

My use case, I wanted to install bower. I couldn't get it to install globally, but the following snippet of my YAML worked:

    - glenmaddern/npm-install:
      options: bower@~0.x
    - script:
        name: Bower install
        code: |
          node_modules/bower/bin/bower install