Add method to get Flysystem driver.

[reinink]

A very simple addition to the new FilesystemAdapter which allows access to underlying Flysystem driver. This is especially helpful when working with libraries that implement the League\Flysystem\FilesystemInterface interface. A simple example:

$this->app->singleton('League\Glide\Server', function ($app) {

    $filesystem = $this->app->make('Illuminate\Contracts\Filesystem\Filesystem');

    return League\Glide\Factories\Server::create([
        'source' => $filesystem->getDriver()
        // etc
    ]);
});

[reinink]

I'm an idiot, I made the method protected. Give me a few minutes and I'll send an update PR. Sorry about that Taylor!