Add virtualenv to prompt, if it exists

With my starting to get deeper into Python development, I noticed an
issue with the virtualenv, where the current virtualenv was not set on
activation. I traced this to the fact that PS1 is always reset in the
prompt_header function, therefore, I need to add the virtual environment
details into prompt_header

This change checks if the virtual environment is set, if so, it adds the
current virtual environment to the prompt in magenta color.

zsh/prompt.zsh

@@ -25,6 +25,13 @@ function prompt_header
     # Add current working directory (green color)
     # This replaces the smartwd functionality with zsh conditionals
     prefix="$prefix [%F{green}%(6~#%-3~/.../%2~#%~)%f]"
+
+    # Add the current virtualenv, if present
+    if [ -n "$VIRTUAL_ENV" ]
+    then
+        prefix="$prefix Py(%F{magenta}${VIRTUAL_ENV##*/}%f)"
+    fi
+
     # Add the status of the last command, but only if it failed
     prefix="$prefix %(?..%F{red}!! %? !!%f)"