Restore prompt_subst setopt when rendering prompt

[mafredri]

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.

---

The was also one other way to fix this, to use anonymous functions in both places where we print the preprompt. Either solution is good imo, anonymous perhaps slightly cleaner (and doesn't depend on zsh/parameter) but I decided to go with this approach, mainly for the reason that I don't know when anonymous functions were introduced into zsh.

Example of anonymous version:

    if [[ "$1" == "precmd" ]]; then
        () {
            setopt local_options no_prompt_subst
            print -P "\n${preprompt}"
        }
    else
        ...
        # modify previous preprompt
        () {
            setopt local_options no_prompt_subst
            print -Pn "${clr_prev_preprompt}\e[${lines}A\e[${COLUMNS}D${preprompt}${clr}\n"
        }