Evaluate $PROMPT at render time (#274)

This allows $PURE_PROMPT_SYMBOL to be modified at any point.

pure.zsh

@@ -368,7 +368,7 @@ prompt_pure_setup() {
 	[[ $UID -eq 0 ]] && prompt_pure_username=' %F{white}%n%f%F{242}@%m%f'
 
 	# prompt turns red if the previous command didn't exit with 0
-	PROMPT="%(?.%F{magenta}.%F{red})${PURE_PROMPT_SYMBOL:-❯}%f "
+	PROMPT='%(?.%F{magenta}.%F{red})${PURE_PROMPT_SYMBOL:-❯}%f '
 }
 
 prompt_pure_setup "$@"