Fix liquidprompt preview
Fix liquidprompt preview

breaking the current prompt after `prompt -p`. Actually liquidprompt is
pretty invasive in the way it is implemented, defining extra
`prompt_on`, `prompt_off` and `prompt_OFF` functions to switch it after
it is executed.

Fixes #112, Closes #165

joshuarubin/dotfiles,joshuarubin/dotfiles,Eriner/zim,joshuarubin/dotfiles,joshuarubin/dotfiles