- consume regexp argument only if present. This fixes a warning reported
in #14.

Tests showing no more warning:

  # find all windows with pid 2593
  % ./xdotool search --pid 2593
  54525953
  54527731
  54525955

  # find all windows with pid 2593 *and* name matches 'tmux'
  % ./xdotool search --all --name --pid 2593 tmux
  54525955

  # find all visible windows with pid 2593
  % ./xdotool search --onlyvisible --pid 2593
  54525955