Kazoo 2460: CDRs chunking

I wonder if we should start creating modules for each doc type, when needed. Something like `kz_doc_cdr` and `kz_doc_device`, for accessing fields like this. Then we could do `kz_doc_cdr:dialed_number(JObj)` and use it in other places (like the CDR whapp or Jonny5).

Thoughts?