Added `options.reload` for reloading the grunt watch task

In #32, a special rule was added for `Gruntfile.js` to reload the file when the configuration changes.
However, as stated in #246, many people split their Gruntfile into separate files, so it would be nice to create a watch target specifically for those files, and reload the Gruntfile when any of them change.

I've added a simple `{Boolean} [options.reload]` parameter that reloads the Grunt task when any file changes.

Here's how I anticipate using it:

```
watch: {
  config: {
    files: [ 'Gruntfile.js', 'config/*.js' ],
    options: { reload: true }
  }
}
```

So I'd like to get feedback ... is this a good approach?