First pass at `split_unknown` function

[krpaulson]

Describe changes

This is a first pass at a "minimum viable product" type version of a split_unknown function. I think a lot of work could be done to increase flexibility of this function, in line with some of the functionality of agg and scale functions in this package.

What issues are related

Related to #22

Checklist

• [x] Have you read the contributing guidelines for ihmeuw-demographics R packages?
• [ ] Have you successfully run devtools::check() locally?
• [x] Have you updated or added function (and vignette if applicable) documentation? Did you update the 'man' and 'NAMESPACE' files with devtools::document()?
• [x] Have you added in tests for the changes included in the PR?
• [x] Do the changes follow the ihmeuw-demographics code style?
• [ ] Do the changes need to be immediately included in a new build of docker-base or docker-internal? If so follow directions in those repositories to rebuild and redeploy the images.
• [ ] Do the changes require updates to other repositories which use this package? If yes, make the necessary updates in those repos, and consider integration tests for those repositories.

Details of PR

I'd like to have a working version of this function good to go soon-ish, but I'm also wondering @chacalle if you're up for spending some time on this to make it more in line with other functions in this package.

[chacalle]

Just need small fixes for the package checks