added via method to notification class

[josephmancuso]

This PR adds the ability to add a via and send method so instead of doing something like:

def show(self, notify: Notify):
    notify.mail(WelcomeNotification, to="email@email.com")
    notify.slack(WelcomeNotification, to="email@email.com")

we can do:

def show(self, notify: Notify):
    notify.via('mail', 'slack').send(WelcomeNotification, to="email@email.com")