code: function rename(file, css, opts) {
      return opts.name({
          dir: path.dirname(file),
          name: path.basename(file, path.extname(file)),
          ext: path.extname(file),
          hash: hash(css, opts.algorithm, opts.trim)
      });
  }
docs: /*
  a function to rename a filename by appending hash value.
  input: ('./file.css', 'a {}', {algorithm: 'sha256', trim: 10})   output: ./file.a1b2c3d4e5.css