Have a method to add tags to the TagSubscriber directly

This is useful to tag responsens when the response object does not yet exist or is not known.

This solves part of #181

@ddeboer do you want me to add a twig extension to tag responses from inside twig templates here? or should we provide that from SymfonyCmfHttpCacheBundle rather? the cmf bundle will extend it to handle doctrine objects in addition to just strings.
while i agree that becoming stateful is not elegant, i don't see a better option. would we need a TagStack or something where we push and pop when subrequests are triggered?