Custom properties missing in GHRepository

[nardew]

Describe the bug
GHRepository class (interface to repos/org/repo_name GitHub endpoint) is missing custom properties. They are mentioned in the GitHub documentation at the root level as

"custom_properties": {
      "type": "object",
      "description": "The custom properties that were defined for the repository. The keys are the custom property names, and the values are the corresponding custom property values.",
      "additionalProperties": true
    }

[bitwiseman]

PR welcome.