Adds service delete command (knative#63)

* Adds service delete command

 This chageset adds `service delete` command, able to delete a service
 at a time.

 Fixes knative#49

```
kn service delete --help
Delete a service

Usage:
  kn service delete <SERVICE_NAME> [flags]

Aliases:
  delete, del, d

Examples:

  # Delete a service 'svc1' in default namespace
  kn service delete svc1

  # Delete a service 'svc2' in 'ns1' namespace
  kn service delete svc2 -n ns1
```

* Removes aliases, PreRunE and addresses review comments

pkg/kn/commands/service.go

@@ -26,5 +26,6 @@ func NewServiceCommand(p *KnParams) *cobra.Command {
 	serviceCmd.AddCommand(NewServiceListCommand(p))
 	serviceCmd.AddCommand(NewServiceDescribeCommand(p))
 	serviceCmd.AddCommand(NewServiceCreateCommand(p))
+	serviceCmd.AddCommand(NewServiceDeleteCommand(p))
 	return serviceCmd
 }

pkg/kn/commands/service_delete.go

@@ -0,0 +1,57 @@
+// Copyright © 2018 The Knative Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package commands
+
+import (
+	"errors"
+	"fmt"
+	"github.com/spf13/cobra"
+	v1 "k8s.io/apimachinery/pkg/apis/meta/v1"
+)
+
+// NewServiceDeleteCommand represent 'service delete' command
+func NewServiceDeleteCommand(p *KnParams) *cobra.Command {
+	serviceDeleteCommand := &cobra.Command{
+		Use:   "delete NAME",
+		Short: "Delete a service.",
+		Example: `
+  # Delete a service 'svc1' in default namespace
+  kn service delete svc1
+
+  # Delete a service 'svc2' in 'ns1' namespace
+  kn service delete svc2 -n ns1`,
+
+		RunE: func(cmd *cobra.Command, args []string) error {
+			if len(args) != 1 {
+				return errors.New("requires the service name.")
+			}
+			client, err := p.ServingFactory()
+			if err != nil {
+				return err
+			}
+			namespace := cmd.Flag("namespace").Value.String()
+			err = client.Services(namespace).Delete(
+				args[0],
+				&v1.DeleteOptions{},
+			)
+			if err != nil {
+				return err
+			}
+			fmt.Fprintf(cmd.OutOrStdout(), "Deleted %s in %s namespace.\n", args[0], namespace)
+			return nil
+		},
+	}
+	return serviceDeleteCommand
+}