Ibuffer: Erase output buffer before shell commands

Ibuffer: Erase output buffer before shell commands

* lisp/ibuf-macs.el (define-ibuffer-op): Add keyword arguments
BEFORE and AFTER; they are forms to run before/after the operation.
* lisp/ibuf-ext.el (ibuffer--maybe-erase-shell-cmd-output):
New defun; if shell-command-dont-erase-buffer is nil, then
erase shell command output buffer.
(ibuffer-do-shell-command-pipe, ibuffer-do-shell-command-file): Use it.