add ability to persist probe window after accept

This was easier than I thought. For one thing, I think it's pretty hard
to leave the probe buffer without accepting or canceling, making the
cleanup autocommands unnecessary. Hopefully there aren't many ways you
can escape the buffer unexpectedly; if there are I might have to re-add
the autocommands and use eventignore when accepting files.

I initally considered additional bindings for functions like
split_and_stay_open, vsplit_and_stay_open, but there just weren't enough
keys. It seems a lot cleaner to make it modal.

I'm ok with how select_appropriate_window behaves. You can't make
arbitrary window layouts with a single probe invocation in persist mode,
but I'll need to use persist mode more to see what makes sense: having
the starting window increment as you add windows versus staying static.

I couldn't come up with a mnemonic key binding, so c-b it is.