Add an option to disable automatic keybinds

I was trying to set my own keybind for `q` when looking at Vader results. I found an easy way to return to the tab I ran the tests for: https://github.com/w0rp/vim/blob/master/after/ftplugin/vader-result.vim#L1

Nothing's ever perfect, and that's good enough for me.

I would like to set my own keybind for `q`, but currently I cannot do this, as Vader always sets its own keybind after everything else here: https://github.com/junegunn/vader.vim/blob/8bbce7dfbd387c9588b7f9e5d14bbae3ea7a45d8/autoload/vader/window.vim#L134

I would like to have an option to disable the automatic keybinds for Vader results. Perhaps `` mappings could be offered, plus a variable for turning the automatic keybinds off.