Uses composer/installers to install directly onto app/Plugin/

Uses [composer/installers](https://github.com/composer/installers) to avoid the complication from having to `git clone` into Plugin and to `composer install` inside of plugin.

User can now simply manage all composer packages at app's `composer.json` and skip `git clone`.


**Note:** 
Once [this installer's feature](https://github.com/composer/installers/pull/46) is released as stable, `"minimum-stability": "dev"` can be removed and simply use version 1.* of composer/installers.