Implement kubectl alpha diff to diff resources

`kubectl alpha diff` lets you diff your resources against live
resources, or last applied, or even preview what changes are going to be
applied on the cluster.

This is still quite premature, and mostly untested.

pkg/kubectl/cmd/alpha.go

@@ -37,6 +37,7 @@ func NewCmdAlpha(f cmdutil.Factory, in io.Reader, out, err io.Writer) *cobra.Com
 	// Alpha commands should be added here. As features graduate from alpha they should move
 	// from here to the CommandGroups defined by NewKubeletCommand() in cmd.go.
 	//cmd.AddCommand(NewCmdDebug(f, in, out, err))
+	cmd.AddCommand(NewCmdDiff(f, out, err))
 
 	// NewKubeletCommand() will hide the alpha command if it has no subcommands. Overriding
 	// the help function ensures a reasonable message if someone types the hidden command anyway.