guix archive: '-f docker' supports package names as arguments.

This allows users to type:

  guix archive -f docker emacs

as was already the case for the 'nar' format.

Reported by David Thompson.

* guix/scripts/archive.scm (%default-options): Add 'format'.
(export-from-store): Dispatch based on the 'format' key in OPTS.
(guix-archive): Call 'export-from-store' in all cases when the 'export'
key is in OPTS.