import/cran: Add input style "specification".

* guix/import/cran.scm (%input-style): New parameter.
(format-inputs): Use it.
* guix/scripts/import/cran.scm (guix-import-cran): Set the %input-style
parameter.
 (%options): Add "--style" option.
* doc/guix.texi (Invoking guix import): Document it.