Generalize Alert Destination

[bh2smith]

With Slack as the current hard-coded alert recipient, it is easy to envision a generalization where we support alternative alert destinations:

• Telegram: with https://pypi.org/project/pyTelegramBotAPI/ or the bot father
• Twitter: https://docs.tweepy.org/en/stable/
• Email: https://realpython.com/python-send-email/

The making such a change should be similar to the abstractions used for Query monitor. Some abstract class with a method post(message) for which each different destination source has its own implementation. We may also want to have a class containing an instance of each destination and an upgrade to the Alert Levels (one for each destination class)

AlertLevel.TELEGRAM, AlertLevel.TWITTER, AlertLevel.EMAIL for which we route the posts to the appropriate place.

[bh2smith]

Twitter Support introduced in #42