Externalize dependencies with a UMD wrap using the standalone option

DEPENDENCY: https://github.com/ForbesLindesay/umd/pull/19

This adds the ability to externalize dependencies using the standalone option

```js
  var bundleStream = browserify('./index.js', {
      standalone: {
        name: 'my-module',
        amd: { deps: ['underscore'] }
      }
    })
    .exclude('underscore')
    .bundle();
```

Note that I am unaware of how active https://github.com/ForbesLindesay/umd is, nor can I guarantee how that PR will go. That said, I think this is an important behavior with the possibility to resolve some browserify issues.