Support arbitrary config splits and environments

[danepowell]

Is your feature request related to a problem? Please describe.
We hardcode the supported config splits, which generally map to environments that we can detect via the EnvironmentDetector. It's a lot of boilerplate code, and people can't easily extend it to add new splits/environments.

Describe the solution you'd like
Add something to EnvironmentDetector to enumerate all "known" environments and whether they are active or not, then build the config split config, and which splits are active, based on whether we are in the respective environment.