Support Multi-Keys

[tospery]

Different JSON use different keys to identify the values that mean same thing. Likes,
First JSON:

{
    "code" : 200,
    "message" : "success"
}

Second JSON:

{
    "code" : 200,
    "msg" : "success"
}

For the 'success' value, you can access the multi-keys object as follows:

func mapping(map: Map) {
    identifier <- map["message|msg", nested: false, delimiter: "|"]
}