New Filters

Add `Filter\DateTime`   filters incoming value into a `\DateTime` object
Add `Filter\DateTimeZone` filters incoming value into a `\DateTimeZone` object
Add `Filter\String::concat` allows prepending and/or appending values to incoming string values.

Primary Use Case:   timestamp validation
```php
$filters = [
    'timestamp' => [
        ['uint'], //ensure value is an unsigned integer
        ['concat', false, '@'], //prepend an '@' character to the integer
        ['date'], //filter the `@timestamp` as a \DateTime
    ],
];
```