Added a new self-update command to easily update the installer

the phar validity should be checked only when ``phar.readonly`` is false in the config, otherwise it will always be reported as corrupted (creating a Phar instance for an existing file is not allowed when the extension is in readonly mode). See the Composer SelfUpdate command where this has been fixed (the PHP-CS-Fixer command is based on an older version of the Composer one)