Make UrlExtension optional

[acelaya]

I have recently migrated two expressive projects from twig to plates, and in both of them I have faced the same problem.

The twigrenderer package includes a set of factories that register a twig extension enabling access to the UrlHelper and the ServerUrlHelper, from the helpers package, but only if both of them are registered in the container.

On the other hand, the platesrenderer package factories force you to register them, otherwise an exception is thrown.

I was not using them in those projects, and I have been forced to install the helpers package and register both services.

Would it make sense to make it optional as in the twigrenderer?

I could provide a PR if you are ok with this idea.

[weierophinney]

Absolutely - go for it!

[acelaya]

Create pull request #25