[feat] enable users to specify keyboard devices through arguments

[EdenQwQ]

example:

pkexec swhkd -D "/dev/input/event0" -D "/dev/input/event1"

use -D or --device to specify a device and the argument can occur multiply.

If the argument is not presented, we'll automatically detect all the keyboard devices.

[Shinyzenith]

Path can change often, maybe a name based identifier would help too?