Allow easy user override of coding system specification

[dogversioning]

Most of our coding tables have some hard coded list of coding systems that we use to select data. We have some general EHR-specific code systems in there, but we anticipate that there will be edge cases in the future we haven't anticipated.

It would be nice to allow for some kind of external configuration to be read that would allow users to define extra coding systems to check for, so that we don't have to cut a release for every new case we discover.