Added support for the new label_format option

Hey guys,

first off, thanks for this bundle! Been using it for a lot of my 'skeleton' projects and especially appreciate the ability to supply my own `variables.less`.

I made a small change to your form-template so that it now supports the new `label_format` option that was added in Symfony 2.6: https://github.com/symfony/symfony/pull/12050. I use this style of label translation a lot in my projects at home and at work, so it would be nice to have this supported in your bundle as well.

I've made sure to use `is defined` tests to keep compatibility with earlier versions. 

Let me know if this needs some fixes/tweaks, happy to modify!

Cheers!