Expose all Nexmo flags in NexmoMessage, i.e. concat-*

[jbtibor]

https://docs.nexmo.com/index.php/sms-api/handle-inbound-message
Some of the missing:
keyword: First word in the message body, typically used with short codes
concat: Set to true if a MO concatenated message is detected
concat-ref: Transaction reference, all message parts will shared the same transaction reference
concat-total: The total number of parts in this concatenated message set
concat-part: The part number of this message within the set (starts at 1)

Or you could have an originalMessage property what would hold all get parameters so that new additions to Nexmo API can be used without updating your library.