config: merge arbitrary map's together

[adamdecaf]

Previously map[K]V types were overwritten, so APP_CONFIG_SECRETS would overwrite APP_CONFIG's map[K]V value. Now we merge the two maps together, which I would argue is the expected behavior. Structs merge together and maps are useful with arbitrary keys.

There are only a small few instances of map[K]V being used in configs and file-saver is the first instance where config-secrets.yml would overwrite part of config.yml.