make the pydf alias optional

pydf is not a true replacement for df, as it cannot stat remote disks.
This alias being a default is inappropriate,
especially without making it very clear in the documentation

modules/utility/init.zsh

@@ -175,12 +175,16 @@ elif (( $+commands[wget] )); then
   alias get='wget --continue --progress=bar --timestamping'
 fi
 
+if zstyle -s ':prezto:module:utility' pydf 'yes'; then
   # Resource Usage
   if (( $+commands[pydf] )); then
     alias df=pydf
   else
     alias df='df -kh'
   fi
+else
+  alias df='df -kh'
+fi
 
 alias du='du -kh'