Add a replacement for `logging.handlers.SMTPHandler` that integrates with Flask-Mail

[doekman]

I've made the log-handler FlaskMailHandler that integrates with Flask-Mail. It solves some problems that logging.handlers.SMTPHandler has:

• It integrates with Flask-Mail configuration
• It can use TLS, even when no credentials are supplied
• You can use text and/or html messages

Would this something that could be added to Flask-Mail? I would be happy to create a pull request.