Fix prompt setup when loaded using zsh prompt function

[belak]

This is very similar to Powerlevel9k/powerlevel9k@f46c1bc

It's based off code in zsh itself. prompt_opts are used when the prompt was loaded via promptinit/prompt and the setopt is a workaround for oh-my-zsh to ensure all the proper prompt options are set.

[denysdovhan]

Wow, didn't know about it. Would it affect users that use $ZSH_THEME with OMZ?

[belak]

Possibly, but probably not. OMZ doesn't end up calling prompt so these options don't get reset. Additionally, I think prompt_subst is set every time the title is updated (I'm not familiar with OMZ very much, as I work on prezto, but this looks relevant: https://github.com/robbyrussell/oh-my-zsh/blob/3705d47bb3f3229234cba992320eadc97a221caf/lib/termsupport.zsh), so at least that will be enabled.

[belak]

I've added some additional background as to why this doesn't show up in OMZ here: zsh-users/antigen#539 (comment)

[denysdovhan]

Thanks for contributing! Merged in #148. A further conversation should continue there.