add defcustom and change default to create new shell without asking

I changed the default here to ask no questions for creating new buffers when there are none to switch to. My reason would be that if I want to switch to one (I press `C-'`) and there is none, of course I want to create one as otherwise I wouldn't have pressed `C-'`. If you think that is not reasonable I would at least like to have the possibility to configure it so I would change the default to how it was before but leave the defcustom in.