default config: show %avail disk space instead of %free

Long description:

The former property is generally more relevant to end users

To apply this patch, use:
curl http://cr.i3wm.org/patch/313/raw.patch | git am

@@ -53,5 +53,5 @@ load {

 }

 disk "/" {

-        format = "%free"

+        format = "%avail"

 }