Update newer.js

I Propose to add the ability to not overwrite config of a task.
There are a tasks like `grunticon` witch need original config (with all files) to work right - so `grunt-newer` will run full task if any file will change.
To ignore changing config (line 120) need to add in never task options `changeConfig: false`.
You can specify `changeConfig` to all task, or to single taks:
```
newer: {
    options: {
        cache: 'app/cache/grunt',
        changeConfig: false
    }
}
```
or
```
newer: {
    options: {
        cache: 'app/cache/grunt',
        grunticon: {
            changeConfig: false
        }
    }
}
```