scripts/contrib: Add graph-tool

scripts/contrib: Add graph-tool

A simple script I put together for getting the paths from one node to
another in a dot graph. This is useful for example in working out why
a particular recipe is getting built in conjunction with dot graph files
produced by bitbake -g.

For example:

$ bitbake -g core-image-minimal
...
$ graph-tool find-paths pn-depends.dot core-image-minimal util-linux
core-image-minimal -> packagegroup-core-boot -> udev -> glib-2.0 -> python-dbus -> dbus-glib -> dbus -> libsm -> e2fsprogs -> util-linux
core-image-minimal -> packagegroup-core-boot -> udev -> glib-2.0 -> python-dbus -> dbus -> libsm -> e2fsprogs -> util-linux
core-image-minimal -> packagegroup-core-boot -> udev -> util-linux

Partially addresses [YOCTO #3362].

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Saul Wold <sgw@linux.intel.com>