Add support for extensible config in contrib modules

It's often difficult to make contrib modules work together. When one
depends on a functionality of another, it is often necessary to expose
lots of low-level functions and hooks and have the user combine these
into a complex configuration that works. This is error-prone, and
arguably a bad UX in general.

This commit presents a simple solution to that problem inspired by
"extensible state": extensible config. It allows contrib modules to
store custom configuration values inside XConfig. This lets them create
custom hooks, ensure they hook into xmonad core only once, and possibly
other use cases I haven't thought of yet.

For more, see the related pull request to xmonad-contrib.

Related: https://github.com/xmonad/xmonad-contrib/pull/547

src/XMonad/Config.hs

@@ -277,6 +277,7 @@ instance (a ~ Choose Tall (Choose (Mirror Tall) Full)) => Default (XConfig a) wh
     , XMonad.handleExtraArgs = \ xs theConf -> case xs of
                 [] -> return theConf
                 _ -> fail ("unrecognized flags:" ++ show xs)
+    , XMonad.extensibleConf     = M.empty
     }
 
 -- | The default set of configuration values itself