More-Safely Set Config Values

Do not overwrite existing config entries. This will prevent accidentally breaking your app because you happened to name a YAML file the same as a possibly unrelated config value.

This will put you in a situation where some values that you might expect to be the contents of your YAML file will be something else, but I think this is a more sane behavior if you integrate conflate into an app where you're already mucking about with the config object before your initializers.