Make --continue as no_argument

This is an improvement for the previous commit, that make --continue
option as no_argument.

Tested: phosphor-gpio-monitor is able to run with or without --continue
        argument, and it it continues to run after key press with this
        argument.

Change-Id: I6f9dc5423e19eace366e382a2408dd143a124915
Signed-off-by: Lei YU <mine260309@gmail.com>
1 file changed