Add back the prompt_sp option for zsh >= 5.4.1

[belak]

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

This was the change which caused it to be reset: https://github.com/zsh-users/zsh/commit/43e55a9bcd2c90124a751f2597d2f33cb6e3c042#diff-bb10d67e7a8561b66a53a805f3c77a40R233

This is the documentation on the option itself: http://zsh.sourceforge.net/Doc/Release/Options.html#Prompting

[belak]

What's the status on this @agnoster ?

[belak]

This is also related to #60, though if this fix goes in, it should work there as well.