[Feature Request] Gamepad hotplugging

[SuperSamus]

Right now, a gamepad is detected only if bsnes is launched while the gamepad is connected to the computer. If the gamepad is connected after that, then it won't be detected, forcing the user to restart bsnes (or reload the input driver).

This is pretty annoying, and it would be useful if there was no need to that it.

(Note I am using Linux, and tested both udev and SDL, I don't know whether the APIs on other systems like Windows support it. Also, ares does support hotplugging on udev and SDL.)