Add additional prompt for co-authors

[p-m-p]

Hi, as per my comment on commitizen/cz-cli#678 I have created a simple wrapper around this adapter with an additional (optional) prompt for adding co-authors. If defaultCoAuthors config property or CZ_CO_AUTHORS environment variable are set then the prompt will be enabled and ask if you would like to add Co-authored-by: message trailers used by Github, Gitlab etc to attribute team members when pair/mob programming.

I would be happy to contribute the change/prompt here instead of having a separate adapter if this is something that would be accepted?

Many thanks