Add Pushover critical alert/high priority support

I'm using Pushover and I'd love to have the option to set some alerts as critical (for example the window down or a plugin reminder)

Just adding the

priority - send as -2 to generate no notification/alert, -1 to always send as a quiet notification, 1 to display as high-priority and bypass the user's quiet hours, or 2 to also require confirmation from the user

Parameter should do the trick (and an extra option for the Pushover check boxes in Schedules)