Change the task to a multitask

Allow the task to be used for different setups in the same gruntfile.js.

`options` will contain the shared options for the tasks, but can still be run by simply running the `requirejs_obfuscate` task.

But by making it a multitask, you can add "sub tasks" like shown below.

```javascript
grunt.initConfig({
  requirejs_obfuscate: {
    options: {
        dir: 'www/js',
        salt: 'salt',
        root: 'com',
        length: 6,
        quotes: 'double',
        exclude: [
            'lib/require.js',
            'lib/jquery-2.0.3.js'
        ],
        output: false
    },
    beta: {
        dir: 'www/js-beta',
        salt: 'myBetaSalt'
    }
  },
})
```

The options in the "sub tasks" will then override the options in `options` so you can run the `requirejs_obfuscate:beta` to use those other options.