Add support for customising the rule name for use with Laravel 6/7

[digistorm-developer]

This allows the package to be used in Laravel 6/7 by allowing customisation of the rule name.
In Laravel 6 a password validation rule was added with different functionality: https://laravel.com/docs/6.x/validation#rule-password

To use the package with Laravel 6/7 publish the config file:

php artisan vendor:publish --provider="PasswordPolicy\Providers\Laravel\PasswordPolicyServiceProvider"

Then specify a different name for the rule in the config e.g. password_policy and it will fix #7.