Add json flag

The json flag returns the output as a JSON document like so:

```json
{
  "addresses": [
    "172.31.92.102",
    "172.31.34.208",
    "172.31.13.215"
  ]
}
```

cmd/discover/main.go

@@ -16,8 +16,10 @@ import (
 func main() {
 	var quiet bool
 	var help bool
+	var json bool
 	flag.BoolVar(&quiet, "q", false, "no verbose output")
 	flag.BoolVar(&help, "h", false, "print help")
+	flag.BoolVar(&json, "json", false, "output response as json")
 	flag.Parse()
 
 	d := &discover.Discover{}
@@ -42,5 +44,10 @@ func main() {
 	if err != nil {
 		l.Fatal(err)
 	}
-	fmt.Println(strings.Join(addrs, " "))
+	switch addrs.(type) {
+	case string:
+		fmt.Println(addrs)
+	case []string:
+		fmt.Println(strings.Join(addrs.([]string), " "))
+	}
 }

discover.go

@@ -3,6 +3,7 @@
 package discover
 
 import (
+	"encoding/json"
 	"fmt"
 	"log"
 	"sort"
@@ -147,7 +148,7 @@ func (d *Discover) Help() string {
 // Addrs discovers ip addresses of nodes that match the given filter criteria.
 // The config string must have the format 'provider=xxx key=val key=val ...'
 // where the keys and values are provider specific. The values are URL encoded.
-func (d *Discover) Addrs(cfg string, l *log.Logger) ([]string, error) {
+func (d *Discover) Addrs(cfg string, l *log.Logger) (interface{}, error) {
 	d.once.Do(d.initProviders)
 
 	args, err := Parse(cfg)
@@ -176,5 +177,16 @@ func (d *Discover) Addrs(cfg string, l *log.Logger) ([]string, error) {
 		return p.Addrs(args, l)
 	}
 
-	return p.Addrs(args, l)
+	addrs, err := p.Addrs(args, l)
+	if _, ok := args["json"]; ok {
+		r := map[string][]string{
+			"addresses": addrs,
+		}
+		b, err := json.MarshalIndent(r, "", "  ")
+		if err != nil {
+			return nil, fmt.Errorf("discover: unable to marshal response")
+		}
+		return string(b), err
+	}
+	return addrs, err
 }