Restore prompt_subst setopt when rendering prompt (#231)

We store the user setting for `prompt_subst` before changing it in the
local scope of `prompt_pure_preprompt_render`. This allows us to restore
it when we render the real prompt (`PS1`) through `zle .redraw-prompt`.

Fixes #230.

pure.zsh

@@ -123,7 +123,10 @@ prompt_pure_string_length_to_var() {
 }
 
 prompt_pure_preprompt_render() {
-	# make sure prompt_subst is unset to prevent parameter expansion in prompt
+	# store the current prompt_subst setting so that it can be restored later
+	local prompt_subst_status=$options[prompt_subst]
+
+	# make sure prompt_subst is unset to prevent parameter expansion in preprompt
 	setopt local_options no_prompt_subst
 
 	# check that no command is currently running, the preprompt will otherwise be rendered in the wrong place
@@ -192,6 +195,11 @@ prompt_pure_preprompt_render() {
 		# modify previous preprompt
 		print -Pn "${clr_prev_preprompt}\e[${lines}A\e[${COLUMNS}D${preprompt}${clr}\n"
 
+		if [[ $prompt_subst_status = 'on' ]]; then
+			# re-eanble prompt_subst for expansion on PS1
+			setopt prompt_subst
+		fi
+
 		# redraw prompt (also resets cursor position)
 		zle && zle .reset-prompt
 	fi
@@ -324,6 +332,8 @@ prompt_pure_setup() {
 
 	zmodload zsh/datetime
 	zmodload zsh/zle
+	zmodload zsh/parameter
+
 	autoload -Uz add-zsh-hook
 	autoload -Uz vcs_info
 	autoload -Uz async && async