TokenForHost returns string "oauth_token" as source when config is read from file #94

TokenForHost source return value should be a value like GH_TOKEN or GITHUB_TOKEN when the token comes from an environment variable, or a file path (e.g. /path/to/hosts.yml) when the value comes from a file. However, a static string oauth_token is returned instead of a file name:

Since oauth_token source is not useful to find out where a value comes from, printing the file name would be better.

Ref. #44