Add a binding for switching through windows backwards

Still not quite happy with this as for some reason it changes
workspaces rather than only cycling through windows in the current
workspace (which is what =C-x o= does), but it's better than nothing.

config.org

@@ -36,6 +36,19 @@ needs to be set up to install them if they aren't already.
     (scroll-bar-mode -1)
   #+end_src
 
+  It's nice to have an inverse of =C-x o= for switching between
+  windows. =other-window=, the function that =C-x o= is bound to,
+  takes an argument =COUNT= that determines how many windows it skips
+  forwards so we can simply pass -1 to =other-window= in a lambda and
+  bind to that:
+
+  #+begin_src emacs-lisp
+    (global-set-key (kbd "C-x O")
+                    (lambda ()
+                      (interactive)
+                      (other-window -1)))
+  #+end_src
+
 ** Colour Scheme
    Currently using =spacemacs-theme='s light variant, but I prefer a pure
    white background to the off-white it has by default.