Add context.matches support in app.message handlers #37

Description

As with Bolt for jS, the app.message handler should provide context.matches to listener functions.

app.message(/^(hi|hello|hey).*/, async ({ context, say }) => {
  // RegExp matches are inside of context.matches
  const greeting = context.matches[0];

  await say(`${greeting}, how are you?`);
});

https://github.com/slackapi/bolt-js/blob/%40slack/bolt%402.2.3/docs/_basic/listening_messages.md#using-a-regexp-pattern

Category (place an x in each of the [ ])

• slack_bolt.App and/or its core components
• slack_bolt.async_app.AsyncApp and/or its core components
• Adapters in slack_bolt.adapter

Requirements (place an x in each of the [ ])

• I've read and understood the Contributing guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.
• I've searched for any related issues and avoided creating a duplicate issue here.