The most flexible programmable editor is probably Acme though.
ACME's plumber is very cool, and allows very easy customization of how some things work. Much easier than these things typically are in Emacs. But this behavior is implementable in Emacs (I wish it were standard), and lots of things that are hard-coded in ACME are fully customizable in Emacs.
Interestingly, the first Emacs "plumber" which I have found after you wrote it literally uses Acme's plumber: https://github.com/alcah/sink.el
I wish Emacs had 9P support though.