Facebook sender_action support ("is typing" indicator)

[luisfarzati]

Implements sender_action parameter from the API as in: https://developers.facebook.com/docs/messenger-platform/send-api-reference#payload

Also adds convenience method replyWithTyping, following same convention than in Slackbot.

 +bot.startTyping(message, function () {
 +  // do something here, the "is typing" animation is visible
 +});
 +
 +bot.stopTyping(message, function () {
 +  // do something here, the "is typing" animation is not visible
 +});
 +
 +bot.replyWithTyping(message, 'Hello there, my friend!');
 +```

[benbrown]

This is getting merged into 0.4! Thank you!