`z -I` had 2 issues:
- it used `subprocess.check_output()`, but didn't UTF-8 decode the string (like the `if mode` statement's `else` branch does on row 80)
- it wasn't actually interactive, like `-I` is supposed to be (subprocess.check_output()` is not interactive)
I'm not sure why `-I` mode was originally treated differently from e.g. `-i` mode, but the same code works for both.
I took the `redraw_window` console command from the original `-I` mode code, because without it the screen updates a bit slower, causing an ugly "flash".
