Added functionality for setting custom messages into current translator. Also updated documentation.

sorry this was same as in https://github.com/auraphp/Aura.Filter/blob/develop/src/Aura/Filter/Rule/EqualToField.php#L34-L40  https://github.com/auraphp/Aura.Filter/blob/develop/src/Aura/Filter/Rule/EqualToValue.php#L34-L40  .