Allow config to be reset (removes memoized values)

Hi, I think this might be helpful to do multiple runs within one scrypt/process. e.g:

    AssetSync.config.prefix = "assets"
    AssetSync.sync
    AssetSync.reset_config!
    AssetSync.config.prefix = "deprecated_assets"
    AssetSync.sync

Let me know what you think, thank you!