Add function to parse Figment-based configuration

[tlater-famedly]

Basically all our services now use figment for configuration, and all more or less write the same bits of code.

There is however one gnarly footgun in figment that we don't handle consistently, and the details of logging setup when a LogLevel is part of the configuration are also easy to miss. We should also assert that configuration priority (config before env before CLI) is done correctly in every repository, which we simply do not.

We should add a simple config parsing helper to this repository. It could even call process::exit() so that we do not need to care about error cases in config parsing elsewhere at all.

This would help by making configuration parsing take far less attention to detail, and increase consistency of how our services behave in front of end users.