Add support for skipping VIRTUALENVWRAPPER init in modules/python (#598)

* Support skip-virtualenvwrapper-init zstyle in python module to avoid auto-sourcing of virtualenvwrapper.sh
* Don't overwrite the WORKON_HOME env var if the user has already set it

modules/python/init.zsh

@@ -34,10 +34,11 @@ if (( ! $+commands[python] && ! $+commands[pyenv] )); then
   return 1
 fi
 
-# Load virtualenvwrapper into the shell session.
-if (( $+commands[virtualenvwrapper.sh] )); then
+# Load virtualenvwrapper into the shell session, unless requested not to
+zstyle -t ':prezto:module:python' skip-virtualenvwrapper-init
+if (( $? && $+commands[virtualenvwrapper.sh] )); then
   # Set the directory where virtual environments are stored.
-  export WORKON_HOME="$HOME/.virtualenvs"
+  export WORKON_HOME="${WORKON_HOME:-$HOME/.virtualenvs}"
 
   # Disable the virtualenv prompt.
   VIRTUAL_ENV_DISABLE_PROMPT=1