Add optional spawnOpts

Add optional spawnOpts object so the opts in child_process.spawn([arg], [opts]) can be configured in the gruntFile. This provides the gruntFile with more control over how the child_process is spawned. An example might be changing the current working directory (cwd):

```js
nodemon: {
  server: {
    options: {
      file: 'server.js',
      args: ['NODE_ENV=' + environment.NODE_ENV],
      ignoredFiles: ['README.md','node_modules/**'],
      watchedExtensions: ['js'],
      watchedFolders: ['server'],
      debug: true,
      delayTime: 1,
      spawnOpts: {
        cwd: __dirname
       }
     }
   }
}
```

Read more about child_process.spawn([arg], [opts]) [here](http://nodejs.org/api/child_process.html#child_process_child_process_spawn_command_args_options).

Documentation has been updated.