Prevent parameter expansion in preprompt

By unsetting `prompt_subst` for the `prompt_pure_preprompt_render`
function we can avoid e.g. git branch names with a `$`-sign to be
expanded as parameters.

Fixes #224 and #225 (as a result of 6cbd49a).

pure.zsh

@@ -123,6 +123,9 @@ prompt_pure_string_length_to_var() {
 }
 
 prompt_pure_preprompt_render() {
+	# make sure prompt_subst is unset to prevent parameter expansion in prompt
+	setopt local_options no_prompt_subst
+
 	# check that no command is currently running, the preprompt will otherwise be rendered in the wrong place
 	[[ -n ${prompt_pure_cmd_timestamp+x} && "$1" != "precmd" ]] && return