simple theme: indication of privileges (#3728)

I found this quite annoying not being in this theme after switching from gentoo-theme, it's helpful and it doesn't detract from simplicity.

themes/simple.zsh-theme

-PROMPT='%{$fg[green]%}%~%{$fg_bold[blue]%}$(git_prompt_info)%{$reset_color%} '
+PROMPT='%(!.%{$fg[red]%}.%{$fg[green]%})%~%{$fg_bold[blue]%}$(git_prompt_info)%{$reset_color%} '
 
 ZSH_THEME_GIT_PROMPT_PREFIX="("
 ZSH_THEME_GIT_PROMPT_SUFFIX=")"