cmd/kpod/images.go: Add structured format ouput

For kpod images, we need to output in JSON format so that consumers
(programatic) have structured input to work with.

kpod images --format json

Signed-off-by: baude <bbaude@redhat.com>

cmd/kpod/formats/formats.go

@@ -0,0 +1,54 @@
+package formats
+
+import (
+	"encoding/json"
+	"fmt"
+	"github.com/pkg/errors"
+	"os"
+	"text/template"
+)
+
+// Writer interface for outputs
+type Writer interface {
+	Out() error
+}
+
+// JSONstruct for JSON output
+type JSONstruct struct {
+	Output []interface{}
+}
+
+// StdoutTemplate for Go template output
+type StdoutTemplate struct {
+	Output   []interface{}
+	Template string
+}
+
+// Out method for JSON
+func (j JSONstruct) Out() error {
+	data, err := json.MarshalIndent(j.Output, "", "    ")
+	if err != nil {
+		return err
+	}
+	fmt.Printf("%s\n", data)
+	return nil
+}
+
+// Out method for Go templates
+func (t StdoutTemplate) Out() error {
+
+	tmpl, err := template.New("image").Parse(t.Template)
+	if err != nil {
+		return errors.Wrapf(err, "Template parsing error")
+	}
+
+	for _, img := range t.Output {
+		err = tmpl.Execute(os.Stdout, img)
+		if err != nil {
+			return err
+		}
+		fmt.Println()
+	}
+	return nil
+
+}