Add getParameter() to IncomingMessage class to enable deep linking

Some drivers have a deep linking mechanism, that allows for passing additional parameters to the bot on startup. It could be a command that launches the bot — or an auth token to connect the user's Telegram account to their account on some external service.

```
class IncomingMessage
{
/**
* @return string
*/
public function getParameter() {
}
}
```

This function MUST return the parameter passed in the link.

Note:
1. **Telegram**: https://core.telegram.org/bots#deep-linking
2. **Facebook**: https://developers.facebook.com/docs/messenger-platform/discovery/m-me-links