Feature request: Option to prevent exiting goyo mode on quit/bd etc

I like to stay in goyo until close all buffers or manually call `Goyo()` to change modes.

I was hoping to be able to prevent goyo from intercepting `:q` and `:bd` and calling `goyo_exit()`, but I couldn't figure out how to do this.

The best I could do was to write a `s:goyo_leave()` function like this:

```
function! s:goyo_leave()
call goyo#execute(0, g:goyo_width . "x" . g:goyo_height)
endfunction
```
Whoops, submitted to soon, continuing:

It re-enters goyo whenever it tries to exit. This is suboptimal, since once you enter goyo, you can't leave.

Would it be possible to provide a boolean setting which allows preventing the auto-off functionality?

Something like: `g:goyo_auto_off` which defaults to `true`?