emacs: Add 'guix-generations-by-time' command.

* emacs/guix-main.scm (find-generations): Add finding generations by time.
* emacs/guix-utils.el (guix-read-date): New procedure.
* emacs/guix-base.el (guix-messages): Add new messages.
* emacs/guix.el (guix-generations-by-time): New command.
* doc/emacs.texi (emacs Commands): Document it.