Add a help property to rule

[ghost]

The rule object has a helpUri property. For scenarios where a rule's help is not available via a URI, it would be nice if rule had a help property whose value was an object specifying the MIME type and text of the help content, for example:

rules: {
  "EX0001": {
    "id": "EX0001",
    "name": "DontDoBadThings",
    "help": {
      "mimeType": "text/markdown",
      "text": "# Rule EX0001: Don't do bad things\nIt is _really_ not nice to do bad things."
    }
  }
}