This is a new variable that will need to be set on all new prompts and
is not backwards compatible with custom prompts that are not prezto
managed, but use prezto's editor-info functionality. Updated the
README.md with additional information for themes.
This was a change in zsh 5.4.1 where prompt_sp is now reset on calls to
prompt, unlike previous behavior where it was set by default. This
restores the previous default behavior.
Refs #1423, but is not a complete fix because we need to wait for
external prompts to update as well.
