-- opts.filter (RegExp): a RegExp which could look like this: /^CVS$|.idea$|.DS_Store$/. If added, every file or directory which matches the RegExp will be ignored. In this example every "CVS" directory and all ".idea" and ".DS_Stores" files would be ignored.
-- opts.filter (Boolean): If set tu true, the RegExp in opts.filter will be used as a whitelist. Every file or directory which DOESN'T match opts.filter will be ignored.
