Hide virtualenv when explicitly disabled by the user (#381)

When `VIRTUAL_ENV_DISABLE_PROMPT` is set (to true) by the user, Pure
will not display the virtualenv in the prompt. Setting this manually
likely means the user does not care.

The user can, at any point, run `export VIRTUAL_ENV_DISABLE_PROMPT=1`
and the virtualenv will disappear. Likewise,
`unset VIRTUAL_ENV_DISABLE_PROMPT` will bring it back.

Closes #350.

A big thanks to classner and pfrybar for their efforts and reference
implementations, inspiring this one.

Author: mafredri

pure.zsh

@@ -85,6 +85,11 @@ prompt_pure_preexec() {
 
 	# shows the current dir and executed command in the title while a process is active
 	prompt_pure_set_title 'ignore-escape' "$PWD:t: $2"
+
+	# Disallow python virtualenv from updating the prompt, set it to 12 if
+	# untouched by the user to indicate that Pure modified it. Here we use
+	# magic number 12, same as in psvar.
+	export VIRTUAL_ENV_DISABLE_PROMPT=${VIRTUAL_ENV_DISABLE_PROMPT:-12}
 }
 
 # string length ignoring ansi escapes
@@ -171,9 +176,15 @@ prompt_pure_precmd() {
 	# preform async git dirty check and fetch
 	prompt_pure_async_tasks
 
-	# store name of virtualenv in psvar if activated
+	# Check if we should display the virtual env, we use a sufficiently high
+	# index of psvar (12) here to avoid collisions with user defined entries.
 	psvar[12]=
-	[[ -n $VIRTUAL_ENV ]] && psvar[12]="${VIRTUAL_ENV:t}"
+	# When VIRTUAL_ENV_DISABLE_PROMPT is empty, it was unset by the user and
+	# Pure should take back control.
+	if [[ -n $VIRTUAL_ENV ]] && [[ -z $VIRTUAL_ENV_DISABLE_PROMPT || $VIRTUAL_ENV_DISABLE_PROMPT = 12 ]]; then
+		psvar[12]="${VIRTUAL_ENV:t}"
+		export VIRTUAL_ENV_DISABLE_PROMPT=12
+	fi
 
 	# print the preprompt
 	prompt_pure_preprompt_render "precmd"
@@ -430,9 +441,6 @@ prompt_pure_setup() {
 	# Prevent percentage showing up if output doesn't end with a newline.
 	export PROMPT_EOL_MARK=''
 
-	# disallow python virtualenvs from updating the prompt
-	export VIRTUAL_ENV_DISABLE_PROMPT=1
-
 	prompt_opts=(subst percent)
 
 	# borrowed from promptinit, sets the prompt options in case pure was not