[ZSH] Make virtualenvwrapper lazy

zsh/.virtual_env_config.zsh_mac.example

@@ -8,11 +8,11 @@ fi
 export WORKON_HOME=~/python-envs
 export VIRTUALENVWRAPPER_VIRTUALENV=$PREFIX/bin/virtualenv
 mkdir -p $WORKON_HOME
-source $PREFIX/bin/virtualenvwrapper.sh
+source $PREFIX/bin/virtualenvwrapper_lazy.sh
 
 # NOTE: an inherited VIRTUAL_ENV is *not* re-activated here — that happens at the
 # very end of .zshrc so activate captures the fully-built PATH (see the comment
-# there). Sourcing virtualenvwrapper.sh above only defines workon/deactivate/etc.
+# there). virtualenvwrapper initializes when a command such as workon is used.
 
 # Mirror the current venv into a per-pane tmux option so that splitting a pane
 # can carry it into the new pane (.tmux.conf reads #{@venv} with `-e`). Per-pane