Add very basic virtualenv plugin

checks for $VIRTUAL_ENV and provides to basename in a function for the prompt.

plugins/virtualenv/virtualenv.plugin.zsh

+function virtualenv_prompt_info(){
+  local virtualenv_path="$VIRTUAL_ENV"
+  if [[ -n $virtualenv_path ]]; then
+    local virtualenv_name=`basename $virtualenv_path`
+    printf "%s[%s] " "%{${fg[yellow]}%}" $virtualenv_name
+  fi
+}
+
+export VIRTUAL_ENV_DISABLE_PROMPT=1