Support specifying a default duration

[nogweii]

I'd like to use Wi-Fi Timer in a slightly different manner than it seems it was designed for.

Myself, I'll run into a time when the Wi-Fi network sucks, or I know it won't be present and I turn it off. I then forget that I did so, and end up using a ton of data.

I'd like to be able to set a default duration. When I turn off Wi-Fi, I should not be prompted, and the timestamp to turn it on again would be calculated. The notification entry should still be there if I want to delay or cancel it, of course.