Add a "check-config" command that detects incompatible/wrong configuration

This resolves #72 (closed).

• Add the CLI subcommand named check-config
• Add tests for running it against a valid config and configs that are invalid in two different ways.
• Add a utils::init_cli_logging function that sets up stderr-based logging for CLI tools.
• Disable unknown fields on structs responsible for parsing the configuration file.

However, anti-goals: I'd like to avoid doing "validate the world" type things like DNS name resolution (nginx does this, for ex) in this process - build system sandboxes don't necessarily have network access, so would be unable to use the configtest command.