Refactor composer_manager_build_json() to be more dynamic

Currently the composer_manager_build_json method doesn't combine the 'extra' key array from a composer.json file when executing 'drush composer-json-rebuild'. This key can be used to include information for the 'installer-paths' array used to define folder locations for drupal modules/themes, contrib or custom outside of the usual 'vendor' path, as outlined in https://www.drupal.org/docs/develop/using-composer/using-composer-to-man....

For full disclosure, the array would look as follows:

"extra": {
    "installer-paths": {
        "docroot/": ["type:drupal-core"],
        "docroot/sites/all/libraries/{$name}": ["type:drupal-library"],
        "docroot/sites/all/modules/contrib/{$name}": ["type:drupal-module"],
        "docroot/profiles/contrib/{$name}": ["type:drupal-profile"],
        "docroot/sites/all/themes/contrib/{$name}": ["type:drupal-theme"],
        "drush/{$name}": ["type:drupal-drush"],
        "docroot/sites/all/modules/custom/{$name}": ["type:drupal-custom-module"],
        "docroot/sites/all/themes/custom/{$name}": ["type:drupal-custom-theme"]
    }
},

Patch will be added shortly.

Edit - patch added.