Add source and cache path prefix option

[reinink]

Currently Glide accepts an instance of League\Flysystem\Filesystem for both the source and cache options. And while this works amazing, I'm worried there are situations where it would be desirable to also define a path prefix (folder) for each. This would mostly happen in situations where you only have a single instance of League\Flysystem\Filesystem available to you.

return League\Glide\Factories\Server::create([
    'source' => $filesystem,
    'cache' => $filesystem,
    'source_path_prefix' => 'images',
    'cache_path_prefix' => 'images/.cache'
]);

[reinink]

This has been added: e936fbf