Add a method to get the twig object used in a template.

This allows template users to use their own twig extensions if they want, while also allowing us to remove the "twigInit" hook. Hooks come at a price, and it doesn't make much sense to use them in this case, as they would only be useful if a module wants to add a twig extension even if the code instantiating SimpleSAML_XHTML_Template does not belong to that module. This could lead to unexpected behaviour (i.e. a module adding a hook that creates trouble for the templates defined in another module), so given the lack of use cases supporting the hook and the possible negative consequences implied, it's better to remove it.