graph: Support package transformation options.

* guix/scripts/graph.scm (%options): Append %TRANSFORMATION-OPTIONS.
(show-help): Call 'show-transformation-options-help'.
(guix-graph): Call 'options->transformation' and use it.
* tests/guix-graph.sh: Add test.
* doc/guix.texi (Invoking guix graph): Document it.