Add back the prompt_sp option for zsh >= 5.4.1
Add back the prompt_sp option for zsh >= 5.4.1

In 5.4.1, this option was reset between prompts, so to retain the
previous default behavior, this should be added back.

dritter/powerlevel9k,bhilburn/powerlevel9k,dritter/powerlevel9k,bhilburn/powerlevel9k,natemccurdy/powerlevel9k,natemccurdy/powerlevel9k