Add shuffle filter for Twig

This is related to an issue I opened some days ago regarding the possibility to have such filter at https://github.com/fabpot/Twig/issues/1035

There are quite a bunch use cases for it, but I don't know if you will consider to add it to core.

As it has been quite easy to implement, I have decided to send a PR with it, no worries if you find it to be rejected.

Thinks missing if accepted:

- Add a note in which Twig version this would be added

Waiting for your comments