`PRAETORIAN_RESET_SENDER` might be an optional configure argument?

[imoyao]

https://github.com/dusktreader/flask-praetorian/blob/c23d10e0d6e34b2b3102b9b71e48f006b8397467/flask_praetorian/base.py#L243

    self.reset_sender = app.config.get(
        "PRAETORIAN_RESET_SENDER",
    )

might can be replace by:

...
DEFAULT_SENDER = app.config.get(
                "DEFAULT_SENDER"
            )
...
    self.reset_sender = app.config.get(
        "PRAETORIAN_RESET_SENDER",
        DEFAULT_SENDER 
    )

IPO,the only DEFAULT_SENDER can meet the needs of sending mail. If the application need to use different mail sender, they can further set them separately?

[dusktreader]

It's a good thought! I'll look into it when I have some cycles to work on praetorian.