Configuration namespacing for different authUrls

[tjaneczko]

Tokens should be saved and read in different configuration namespaces for different authUrls so that multiple GitHub sources can be used on the same machine without needing to delete the config file. This could be accomplished by using the auth URL when reading or writing to the config as follows:

{
  "https://ghe.example.com/api/v3": {
    "user": "enterprise_username",
    "token": "enterprise_token"
  }
}

github.com credentials could either continue to be in the root object for backwards compatibility, or namespaced as well.