Add installer paths for drupal custom modules and themes

[AlexSkrypnyk]

Add install paths for types drupal-custom-modules and drupal-custom-themes, otherwise they are installed by composer/installers in themes/custom/{$name} or modules/custom/{$name}.

Use case:

composer create-project drupal-composer/drupal-project:8.x-dev some-dir --no-interaction
composer require vendor/drupal-install-profile

See #517

[normanlolx]

      "web/modules/custom/{$name}": ["type:drupal-custom-module"],
      "web/profiles/custom/{$name}": ["type:drupal-custom-profile"],
      "web/themes/custom/{$name}": ["type:drupal-custom-theme"]