Ensure external prompts also set the prompt_sp option

[belak]

There's some backstory in #1423, but essentially that option needs to be in prompt_opts now.

• [ ] agnoster (PR here: https://github.com/agnoster/agnoster-zsh-theme/pull/46)
• [ ] powerline (PR here: https://github.com/davidjrice/prezto_powerline/pull/11)
• [x] pure (PR here: https://github.com/sindresorhus/pure/pull/338) - this is actually fine, as it doesn't use cr

[belak]

Small update, this is also related to another issue (#1426).