Implement a basic mapper

This should now be ready to merge. For details, see inline docs and the brief integration test at: https://github.com/dpla/KriKri/pull/23/files#diff-c139162c30d9695d1f73d5b6881e4348R4 

I'll compile a longer commit message on squash.
The key piece of documentation about the interaction is here: https://github.com/dpla/KriKri/pull/23/files#diff-043425efa8c79859d1624358142a9a5aR23

From a user's perspective, I think we'll want to put this somewhere else.  As long as mappings are a thing the tech team writes, this could live on the 'cheat sheet' I was thinking of creating.  When it moves to UI, we'll need some UI documentation and training materials.