hotplug: support for interval commands

This allows executing code with a given interval. As every command, it
can be assign to any uevent.

Intervals may be useful for counting elapsed time since some action. It
allows e.g. indicating that button has been pressed for some time. This
is useful to let user know he can already release the button.

Signed-off-by: Rafał Miłecki <zajec5@gmail.com>