configuration from environment variables

There should be a function to generate a configuration from environment variables. Overriding configuration in the environment is handy for
- scripts that cannot create a configuration file (or cannot edit an existing one) and cannot change the command line, and
- modules that want to read configuration but do not control `sys.argv` (e.g. logging and security modules).

I believe Confit already supports an optional environment variable naming a configuration path. That's good, but I'd extend it to be a colon-separated path (like PATH, CLASSPATH, or PYTHONPATH) of configurations to load in order.

No files, tests, or entry points are named. Start by locating the existing environment-variable configuration-path handling, then determine how a configuration-generating function would accept a colon-separated list and load the configurations in order; done means the requested environment-based override behavior is implemented and verified.

Written by the indexing model from the issue text.