* modify backup because of make the 'no' choice but puts message 'Done backup'
* create 'backup' method
   * unify that tasks do 'backup'
* delete task 'install'
   * save 'install[file]' task, because default 'install[file]' task is equal 'install' task
* delete 'symlink' task
   * it is unnecessary that 'install' include symlink task
* target files rename ".{filename}"
* modify item method for currect extraction dotfiles