Add twig variable craue_formflow_wrapper_class

[pulzarraider]

Adding support for craue_formflow_wrapper_class twig variable to customize the wrapped div.

[coveralls]

Coverage remained the same at 99.459% when pulling 5318762 on pulzarraider:craue_formflow_wrapper_class into cb7f4d5 on craue:master.

[craue]

Thank you for working on a PR. Might surrounding the button template with a custom div in userland code be a better approach instead of adding one more variable for a specific case?