Add OptOutType property for subscription management.

[melgish]

Adds new property OptOutType to TwilioMessage class.

This property is set by Twilio API when an incoming message matches one of the configured subscription keywords. If there is a match, Twilio will set this value to either START, STOP, or HELP.

see https://www.twilio.com/docs/messaging/services/tutorials/advanced-opt-out#keeping-track-of-your-users-status for description.