* Extract parsing of value reference '!' in settings into a function
* Allow to specify full file name in value reference '!'
This `network_key: '!secret network_key'` is really confusing because
it looks like a YAML tag named 'secret' and a value "network_key".
When we write `network_key: '!secret.yaml network_key'` instead, it's
a little clearer what's going on. Also, it's more consistent with file
references in 'devices' and 'groups'.