Properly map Content Audit plugin Owner field [enhancement]

[andyzito]

The Content Audit plugin stores a metadata field per post indicating the "owner" of the content. The current user mapping functionality could be easily extended, I think -- and this might even be an opportunity to make that functionality more flexible and/or broader, to include other cases where the user id is stored by a plugin. If the maintainer is open to this enhancement I will be happy to work on a pull request.

[nicholasio]

I'm definitely open to making the user mapping part extensible. I'll be happy to review a PR.