[TwigBundle] Make twig extension handle custom template escaping guesser

I wanted to add a custom template escaping guesser for twig like in http://twig.sensiolabs.org/doc/recipes.html#using-the-template-name-to-set-the-default-escaping-strategy 

This pull request allows you register a service id and a method as a custom guesser:

    twig:
        autoescape: my_service:guess

Documentation is missing for this PR and the unit tests are not that good. I´d work on it if this PR has any chance of getting merged?

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | none
| License       | MIT
| Doc PR        | (missing)