Make the virtualenv plugin themable

plugins/virtualenv/virtualenv.plugin.zsh

 function virtualenv_prompt_info(){
-  if [[ -n $VIRTUAL_ENV ]]; then
-    printf "%s[%s] " "%{${fg[yellow]}%}" ${${VIRTUAL_ENV}:t}
-  fi
+  [[ -n ${VIRTUAL_ENV} ]] || return
+  echo "${ZSH_THEME_VIRTUALENV_PREFIX:=[}${VIRTUAL_ENV:t}${ZSH_THEME_VIRTUALENV_SUFFIX:=]}"
 }
 
 # disables prompt mangling in virtual_env/bin/activate