Add the gitclean task

```js
grunt.initConfig({
    gitreset: {
        task: {
            options: {
                onlyignoredfiles: true,
                nonstandard: true,
                directories: true
            },
            files: {
              src: // Target-specific paths go here.
            }
          }
        }
    },
});
```

Likely needs some help on the naming of the options.
http://linux.die.net/man/1/git-clean