editor/format: rewrite & fix set-formatter!
editor/format: rewrite & fix set-formatter!

+ Updated docstring (and added two more examples)
+ Can now be used with shell command strings or string lists. String
  sublists can have more than 2 elements. Non-string items will be
  interpolated into the string before it. If any non-string item is nil,
  its sublist is omitted entirely.
+ Can now be used to redefine formatters predefined by the format-all
  package, by passing the formatter's name (as a symbol) as the first
  argument.
+ Added :modes property for cases when first argument isn't a major mode
  or list of them (when redefining a formatter).

UndeadKernel/emacs_doom,hlissner/doom-emacs,hlissner/.emacs.d,UndeadKernel/emacs_doom,hlissner/.emacs.d,hlissner/doom-emacs,hlissner/doom-emacs,UndeadKernel/emacs_doom,UndeadKernel/emacs_doom,hlissner/.emacs.d,hlissner/.emacs.d,hlissner/doom-emacs,hlissner/doom-emacs,hlissner/.emacs.d,UndeadKernel/emacs_doom,hlissner/.emacs.d,hlissner/.emacs.d,hlissner/doom-emacs,UndeadKernel/emacs_doom,hlissner/.emacs.d,UndeadKernel/emacs_doom,hlissner/doom-emacs