Add Download command

Download is similar to cat, excepts it download the configuration and
save it as file with detector name as file name on current folder

cli/cmd/download.go

@@ -0,0 +1,67 @@
+/*
+ * Copyright 2020 Amazon.com, Inc. or its affiliates. All Rights Reserved.
+ * Licensed under the Apache License, Version 2.0 (the "License").
+ * You may not use this file except in compliance with the License.
+ * A copy of the License is located at
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ * or in the "license" file accompanying this file. This file 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 cmd
+
+import (
+	entity "esad/internal/entity/ad"
+	"fmt"
+	"os"
+	"path/filepath"
+
+	"github.com/spf13/cobra"
+)
+
+const commandDownload = "download"
+
+//downloadCmd downloads detectors configuration on current working directory
+//based on detector id or name patter
+var downloadCmd = &cobra.Command{
+	Use:   commandDownload + " [flags] [list of detectors]",
+	Short: "Downloads detectors configurations based on id or name pattern",
+	Long: fmt.Sprintf("Description:\n  " +
+		`Downloads detectors configurations based on id or name pattern, use "" to make sure the name is not matched with pwd lists'`),
+	Run: func(cmd *cobra.Command, args []string) {
+		//If no args, display usage
+		if len(args) < 1 {
+			displayError(cmd.Usage(), commandDownload)
+			return
+		}
+		err := printDetectors(WriteInFile, cmd, args)
+		displayError(err, commandDownload)
+	},
+}
+
+//WriteInFile writes detector's configuration on file
+//file will be created inside current working directory,
+//with detector name as file name
+func WriteInFile(d *entity.DetectorOutput) error {
+	cwd, err := os.Getwd()
+	if err != nil {
+		return err
+	}
+	filePath := filepath.Join(cwd, d.Name)
+	f, err := os.Create(filePath)
+	defer func() {
+		f.Close()
+	}()
+	if err != nil {
+		return err
+	}
+	return FPrint(f, d)
+}
+
+func init() {
+	esadCmd.AddCommand(downloadCmd)
+	downloadCmd.Flags().BoolP("name", "", true, "input is name or pattern")
+	downloadCmd.Flags().BoolP("id", "", false, "input is id")
+}