Allow packages to be installed as dev dependencies

[kielabokkie]

Some packages like kielabokkie/jsonapi-behat-extension should be installed as dev dependencies. We should have a separate entry in the skeletor.yml file to list dev dependencies.

[kielabokkie]

This is now possible by passing it as a option in the setup() function of the package:

public function setup()
{
    ...
    $this->setPackageOptions('--dev');
}