preview-tui: make debugging easier
preview-tui: make debugging easier

i've noticed that there's a non-trival amount of issues user face
related to preview-tui. allowing all the null redirection to be easily
switched on/off can make debugging much easier, both for the users and
for us.

jarun/nnn,jarun/nnn,jarun/nnn,jarun/noice,jarun/noice,jarun/nnn