provide a quiet option

This pull request provides a quiet option to suppress output to STDOUT. It can be enabled as follows. 

```js
copy: {
  main: {
    options: {
      quiet: true
    },
    files: [
      {
        expand: true,
        cwd: 'src',
        src: ['**',],
        dest: 'build/'
      }
    ]
  }
}
```