Consider making the per-repository configurations case-insensitive

[AustinTSchaffer]

The README mentions that the keys for per-repository configurations must match the repository short name (repo_code) attribute for the repository that it configures, except the key must be all lowercase, regardless of what's in the (repo_code) attribute for the repository.

We should consider making these checking of these keys case-insensitive, so the repository short name can be pasted directly into the configuration for the plugin, or typed out explicitly using lowercase.