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)
I would personally rather implement:

```
class: Acme\DemoBundle\Filter\EnabledUserFilter
```

This filter could then be properly tested, instead of having the logic live in the configuration and passed off to a SimpleFilter class.

When adding BsonFilter to ODM, we followed ORM's lead. They only had an abstract SQLFilter, which is intended to be extended for application-specific implementations. That's why I'd rather not have a basic BsonFilter implementation in ODM, as I mentioned in ps://github.com/doctrine/mongodb-odm/pull/908.

----

A separate concern I had was supporting non-expressions while letting users opt-in to using expressions if they wanted. DI service configurations, for YAML at least (I'm not sure how XML works), use `=@` as a value prefix to turn on expression evaluation. The implementation we had here would always evaluate the input as an expression. I'm not sure how that'd affect users wanting to pass references to parameters (e.g. `%module.parameter%`).

I didn't mention this earlier, as I think it's a moot point alongside my main argument that users are better served with custom filter classes.