Switch the display logic to display-buffer

[Fuco1]

I think rolling our own is not the correct way to integrate with the rest of Emacs. There are packages out there to manage window layout (such as the amazing shackle) which won't work with this package because it bypasses the display-buffer logic.

I think I can make it such that it behaves the same as now or very closely but using display-buffer internally. This allows users to either use some other package to manage the layout or write their own rules in display-buffer-alist.

For example, I would like the popped buffer to take over the selected window, but I don't see an option for it currently. But I can write a simple rule for display-buffer-alist to do that. This way users can customize the look&feel without shell-pop-el needing to add all the different switches.

@kyagi @syohex as you two seem to be most active here, what do you think?