Add support for title and description on Enums

[john-tipper]

Enums defined at the root level do not support titles or descriptions.

{
        "type" : "string",
        "title": "MyEnumTitle",
        "description": "Some description of my enum.",
        "enum" : ["one", "secondOne", "3rd one"]
}

The above json does not cause either the title or description to be created for the enum. However, the following does generate docs:

{
        "type" : "object",
        "properties" : {
            "myEnum" : {
                "title": "MyEnumTitle",
                "description": "Some description of my enum.",
                "type" : "string",
                "enum" : ["one", "secondOne", "3rd one"]
            }
        }
}

This appears to be because enums are treated differently to objects: they do not apply the TitleRule or Description rule (https://github.com/joelittlejohn/jsonschema2pojo/blob/master/jsonschema2pojo-core/src/main/java/org/jsonschema2pojo/rules/EnumRule.java#L100).