Support Webform composite fields

Providing a first approach.
This one currently does not support fields of type Custom composite which is a lot more complex, as it can contain sub fields that are not of type text. All other types are simply arrays of text fields, so they can be joined together just like it is done with normal fields.

@mrshowerman thank you! I'm willing to merge this, just left two comments to resolve.

Thanks for the feedback, @anybody. I applied your suggested changes.

Great, thank you @mrshowerman!
Merging!

Automatically closed - issue fixed for 2 weeks with no activity.