Adding CLI for the migration tool (#154)

* Initial commit for the corefile-tool cli for the migration tool

* cleanup implementation

* nit

* add fn to get corefile path

* add released command and improve readme

kubernetes/corefile-tool/cmd/removed.go

@@ -0,0 +1,52 @@
+package cmd
+
+import (
+	"fmt"
+
+	"github.com/coredns/deployment/kubernetes/migration"
+
+	"github.com/spf13/cobra"
+)
+
+// NewRemovedCmd represents the removed command
+func NewRemovedCmd() *cobra.Command {
+	removedCmd := &cobra.Command{
+		Use:   "removed",
+		Short: "Removed returns a list of removed plugins or directives present in the Corefile.",
+		Example: `# See removed plugins CoreDNS from v1.4.0 to v1.5.0. 
+corefile-tool removed --from 1.4.0 --to 1.5.0 --corefile /path/to/Corefile`,
+		RunE: func(cmd *cobra.Command, args []string) error {
+			from, _ := cmd.Flags().GetString("from")
+			to, _ := cmd.Flags().GetString("to")
+			corefile, _ := cmd.Flags().GetString("corefile")
+			removed, err := removedCorefileFromPath(from, to, corefile)
+			if err != nil {
+				return fmt.Errorf("error while listing deprecated plugins: %v \n", err)
+			}
+			for _, rem := range removed {
+				fmt.Println(rem.ToString())
+			}
+			return nil
+		},
+
+	}
+	removedCmd.Flags().String("from", "", "Required: The version you are migrating from. ")
+	removedCmd.MarkFlagRequired("from")
+	removedCmd.Flags().String("to", "", "Required: The version you are migrating to.")
+	removedCmd.MarkFlagRequired("to")
+	removedCmd.Flags().String("corefile", "", "Required: The path where your Corefile is located.")
+	removedCmd.MarkFlagRequired("corefile")
+
+	return removedCmd
+}
+
+// removedCorefileFromPath takes the path where the Corefile is located and returns the plugins or directives
+// that have been removed.
+func removedCorefileFromPath(fromCoreDNSVersion, toCoreDNSVersion, corefilePath string) ([]migration.Notice, error) {
+	fileBytes, err := getCorefileFromPath(corefilePath)
+	if err != nil {
+		return nil, err
+	}
+	corefileStr := string(fileBytes)
+	return migration.Removed(fromCoreDNSVersion, toCoreDNSVersion, corefileStr)
+}