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.
I think there's a question about how where we want to write the documentation up for having a `Krikri::Mapper` interact with a `Krikri::Parser` object (or at least from a user's perspective, a "parsed record"). I don't know if that's documentation in the code, or just something like wiki pages that demonstrate usage.