Implement multiTasks and targets if a config

Really useful to add a config for different locations where run composer.

E.g. from my `Gruntfile.js`:
```js
composer: {
  options: {
    flags: [
      "no-dev",
      "optimize-autoloader"
    ]
  },
  yourls: {},
  geoip: {
    options: {
      cwd: 'user/plugins/geoip/'
    }
  }
}
```