Search --pid without regexp_argument

[mjsir911]

Implement pid searching without additional search argument to allow commands like:

$ xdotool search --pid 123 set_window --urgency 1

I don't think it made much sense to search by name and pid, previously you had to do something like this:

$ ./xdotool search --pid 123  name_that_does_not_exist set_window --urgency 1
Defaulting to search window name, class, and classname

Fixes #14

[mjsir911]

@jordansissel is there any interest in getting this merged? Can fix the conflict