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.
The prompt function expects all them hooks to use the following
naming convention:
prompt_<theme>_<hook>
This enables it to remove any added theme specific hooks when the
prompt is changed.
Kaleb Elwert