New 'change' check mode + new 'check_modes' option

Completely removed `check_mode` Scalar option.
Introduced a `check_modes` Array option for replacement.
Introduced a `change` validation mode.
Updated docs.

P.S. Did no handling for using former `check_mode` option as Symfony will throw an InvalidConfigurationException itself. If there's need for more informative exception message like 'This option deprecated, use a new one instead', I can code and submit it in a new PR.