added "silent" option to suppress grunt.log.writeln

This would be highly appreciated. Tell me if there is anything else needed to make this happen.
Thanks.
Logging should be abstracted to a new function to avoid repeating. Something like this:

```js
/**
 * @param {string}
function log(msg) {
    if (!options.silent) {
        grunt.log.writeln(msg);
    }
}
```