Add support for multiple search repositories

Up until now, whalebrew restricts its searches to https://hub.docker.com/u/whalebrew

This commit introduces the ability to customize search organization
under docker hub.

To customize it, place a file like

```
registries:
- dockerHub:
    owner: whalebrew
```

You can add as many search repositories as desired, all of them will be
searched.

Currently, there is no authentication to the search repositories. Only
public images will be retrieved.

A step forward for whaleebrew tap discussed in #102

cmd/search.go

@@ -7,6 +7,7 @@ import (
 	"net/url"
 
 	"github.com/spf13/cobra"
+	"github.com/whalebrew/whalebrew/config"
 )
 
 type imageResult struct {
@@ -30,31 +31,37 @@ var searchCommand = &cobra.Command{
 		if len(args) > 1 {
 			return fmt.Errorf("Only one search term is supported")
 		}
+		for _, registry := range config.GetConfig().Registries {
+			if registry.DockerHub != nil {
+				params := url.Values{}
+				params.Set("page_size", "100")
+				params.Set("ordering", "last_updated")
+				if len(args) > 0 {
+					params.Set("name", args[0])
+				}
+				u := url.URL{
+					Scheme:   "https",
+					Host:     "hub.docker.com",
+					Path:     fmt.Sprintf("/v2/repositories/%s/", registry.DockerHub.Owner),
+					RawQuery: params.Encode(),
+				}
 
-		params := url.Values{}
-		params.Set("page_size", "100")
-		params.Set("ordering", "last_updated")
-		if len(args) > 0 {
-			params.Set("name", args[0])
-		}
-		u := url.URL{
-			Scheme:   "https",
-			Host:     "hub.docker.com",
-			Path:     "/v2/repositories/whalebrew/",
-			RawQuery: params.Encode(),
-		}
-
-		r, err := http.Get(u.String())
-		if err != nil {
-			return err
-		}
-		answer := searchAnswer{}
-		err = json.NewDecoder(r.Body).Decode(&answer)
-		if err != nil {
-			return err
-		}
-		for _, image := range answer.Results {
-			fmt.Printf("%s/%s\n", image.User, image.Name)
+				r, err := http.Get(u.String())
+				if err != nil {
+					return err
+				}
+				answer := searchAnswer{}
+				err = json.NewDecoder(r.Body).Decode(&answer)
+				if err != nil {
+					return err
+				}
+				for _, image := range answer.Results {
+					fmt.Printf("%s/%s\n", image.User, image.Name)
+				}
+			} else {
+				err := fmt.Errorf("Unsupported registry %v. Only docker revistry is supported", registry)
+				return err
+			}
 		}
 		return nil
 	},

config/config.go

@@ -0,0 +1,35 @@
+package config
+
+import (
+	"os"
+	"path/filepath"
+
+	"github.com/spf13/viper"
+	"gopkg.in/yaml.v2"
+)
+
+type DockerHubRegistry struct {
+	Owner string `yaml:"owner"`
+}
+
+type Registry struct {
+	DockerHub *DockerHubRegistry `yaml:"dockerHub"`
+}
+
+type Config struct {
+	Registries []Registry `yaml:"registries"`
+}
+
+func GetConfig() Config {
+	configPath := filepath.Join(viper.GetString("config_dir"), "config.yaml")
+	fd, err := os.Open(configPath)
+	c := Config{}
+	if err == nil {
+		defer fd.Close()
+		yaml.NewDecoder(fd).Decode(&c)
+	}
+	if len(c.Registries) == 0 {
+		c.Registries = []Registry{{DockerHub: &DockerHubRegistry{Owner: "whalebrew"}}}
+	}
+	return c
+}