add debounce feature

Created by: hiroqn

#1304 (closed)

need opt.wait option for debounce in watch function because, fn is wrapped gulp.parallell so we cant use like this.

gulp.watch(glob, debounce(gulp.series(scripts, unitTests), 200));

(https://github.com/gulpjs/gulp/blob/4.0/index.js#L42)

  if (typeof task === 'function') {
    fn = this.parallel(task);
  }

or no wrap fn gulp.parallell