Add method to send message with attributes

[bradbyte]

See ccm-innovation/react-native-twilio-ip-messaging#74.

Basically, we need to expose a method to construct a new message instance and then send that inside the sendMessage call. That way, you don't have to send message and then immediately update with attributes.

[svlaev]

@bradbumbalough we have such functionality: #7
Usage is as simple as:

await this.channel.sendAttributedMessage(text, {
  'foo': 'bar',
  'baz': 'qux'
});

[bradbyte]

@svlaev thanks! yeah I forgot to cross reference the issue with your PR.

[bradbyte]

Fixed in #17.

[bradbyte]

Released in 0.2.0.