Variables Class + Extensions

Proof of concept to see how we could manage the variables via a class in the templates. Should be cleaner to handle these through this proxy class.

Specially for filters, allowing to implement #8 : `$_env->filter('core.default', $_env->filter('text.upper', $myVar));`

**Todos**
- [x] Implement the `Link_VariableInterface`
- [x] Implement the `Link_ExtensionInterface`
- [x] set up a basic implementation
- [x] link it to the parser
- [x] make the variable interface useable via `ArrayAccess`
- [x] Add some tests
- [x] Add some documentation