Mistress::App::Command.pm, example_config.yml: minor fixes
everywhere: replace Config::Any with YAML::XS

Sure, Mistress won't be configurable from XML or INI files, but YAML is nicer
anyway. Config::Any understands YAML too, but it uses YAML::Syck which is too
conservative for light and modern YAML configuration files.
(minor) Mistress::Env::Config: replace croak with confess