kubectl-diff: Simplify interface

The current interface is kind of clunky and not super easy to use, since
you have to specify parameters to specify which versions to diff. Also
the default isn't the most useful setting.

Change the interface by removing all the parameters and force only one
useful use-case, that is: diffing what's currently live against
what would be live if applied.

hack/testdata/pod-changed.yaml

@@ -0,0 +1,10 @@
+apiVersion: v1
+kind: Pod
+metadata:
+  name: test-pod
+  labels:
+    name: test-pod-label
+spec:
+  containers:
+  - name: kubernetes-pause
+    image: k8s.gcr.io/pause:3.0