Filter parameters config with expressions

This pull is related to https://github.com/doctrine/mongodb-odm/issues/899
It allows set filter parameters in config file.
Parse value parameter by [ExpressionLanguage](http://symfony.com/doc/current/components/expression_language/syntax.html)
You can also create custom functions very simply:
```php
class Language extends ExpressionLanguage
{
    protected function registerFunctions()
    {
        // Registering our 'date' function
        $this->register('date', function ($date) {
            return sprintf('(new \DateTime(%s))', $date);
        }, function (array $values, $date) {
            return new \DateTime($date);
        });
    }
}
```
and use it like in the example https://github.com/doctrine/mongodb-odm/issues/899#issuecomment-46082507
```value: date("today")```