Add option for quieted output

As Issue #22 mentions, I thought it would be great if there was a way to suppress output, so I added one.

Here's the configuration:

```javascript
clean: {
    build: {
        src: ['tmp/sample_quiet/*', 'tmp/sample_quiet'],
        quiet: true,
      },
}
```

and the accompanying output:
```shell
Running "clean:quiet" (clean) task
Cleaned 3 items.
```

If you'd like me to squash my commits or change anything, just let me know.

Thanks!