[xmonad] New module for xmonad-contrib: SpawnNamedPipe

Hello Xmonad devs, Attached is a simple module that allows you to spawn a pipe and store it in the xmonad state as a String, Handle pair so that it can be accessed from a different hook from where the pipe was initially created. The initial motivation for this was I wanted to be able to spawn dzen from inside the startupHook and write to the pipe from inside the logHook. I didn't like being forced to start the dzen pipe up in a global namespace so that I could access it from the logHook. This example is detailed in the Haddocks but there are many other use cases. Cheers, Chris Wills