Add ability to add directories to MigrationStorage

[Jackardios]

This PR adds the ability to add directories to MigrationStorage. Using the addDirectory function, we can add additional directories for MigrationStorage. This way we can define a function to register additional directories in the ServiceProvider (like loadMigrationsFrom to register database migrations).

For example:

/**
 * Register elastic migration directories.
 *
 * @param  array|string  $directories
 * @return void
 */
protected function loadElasticMigrationsFrom($directories)
{
    $this->callAfterResolving(\ElasticMigrations\Filesystem\MigrationStorage::class, function ($migrationStorage) use ($directories) {
        foreach ((array) $directories as $directory) {
            $migrationStorage->addDirectory($directory);
        }
    });
}

[babenkoivan]

Hey, sorry but I don't see any real benefits of having migrations in multiple directories. This makes code slower (many additional loops) and less readable. Maybe you can create your implementation of MigrationStorage and register it in the application service provider?