add shortcut '&&=' for preprocessing values only when they exist.

* Use `&&=` to preprocess variables that may or may not exist. Using
  `&&=` will change the value only if it exists, removing the need to
  check its existence with `if`.

    ```Ruby
    # bad
    if something
      something = something.downcase
    end

    # ok
    something = something.downcase if something

    # good
    something = something && something.downcase

    # better
    something &&= something.downcase
    ```