[xmonad] spawn command when no completion is available (if alwaysHighlight is True); changes commandToComplete in Prompt/Shell to complete the whole word instead of using getLastWord

This small patch addresses http://www.haskell.org/pipermail/xmonad/2013-February/013512.html. The new behaviour also affects users that have alwaysHighlight set to False (the default) in the following way: - "xterm -e emacs" autocompleted to e.g. ["emacs","emacs23","emacs-24.2",..] before (using the last word) - "xterm -e emacs" autocompletes to nothing now which I'm not sure is a problem. Best, Carlos