Add ability overwrite translations by passing second argument on Lang::set() method.

Lang::set('en', [
    'thousand' => 'thou',
    'million' => 'mil',
    'billion' => 'bil',
    'trillion' => 'tril',
]);

Add easy way to add another language for package with clear documentation

Number::conv(-1000)

should output -1k

Lang::set() method excepts custom translations. Now it excepts translations for all field, but instead it should overwrite only those translations that we provide.

Rename Conv class to Number and short() method to conv()