Add support for downloading from Asset Packagist

[Kalaiselvan88]

Currently we do not download packages for composer from Asset packagist. This is needed for bower libraries used by Acquia's Lightning profile.

Background: https://www.drupal.org/project/lightning/issues/2911640 and http://lightning.acquia.com/blog/round-your-front-end-javascript-libraries-composer

We need to add a line to composer.json as in above link and use oomphinc/composer-installers-extender to alter path of the downloaded bower libraries (this can be done if needed)

As discussed below can be alternatives.

1. Add support to optionally include asset packagist via a flag
2. Detect if lightning is one of the requirement in composer require and add it automatically.