contrib: add graphviz_overlays.py script

Just uploading this in case it helps other folks in the future.  It's
not particularly high-quality code, but hey, this is contrib, right?

BUG=none
TEST=example invocation from docstring

Change-Id: Ic961c978c379e0b3e0f891458597ebff5e05457c
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/dev-util/+/2445799
Commit-Queue: Jack Rosenthal <jrosenth@chromium.org>
Tested-by: Jack Rosenthal <jrosenth@chromium.org>
Reviewed-by: Chris McDonald <cjmcdonald@chromium.org>