Make downloader type configurable

utils/config.go

@@ -12,6 +12,7 @@ type ConfigStructure struct { // nolint: maligned
 	DownloadConcurrency    int                              `json:"downloadConcurrency"`
 	DownloadLimit          int64                            `json:"downloadSpeedLimit"`
 	DownloadRetries        int                              `json:"downloadRetries"`
+	Downloader             string                           `json:"downloader"`
 	DatabaseOpenAttempts   int                              `json:"databaseOpenAttempts"`
 	Architectures          []string                         `json:"architectures"`
 	DepFollowSuggests      bool                             `json:"dependencyFollowSuggests"`
@@ -87,6 +88,7 @@ var Config = ConfigStructure{
 	RootDir:                filepath.Join(os.Getenv("HOME"), ".aptly"),
 	DownloadConcurrency:    4,
 	DownloadLimit:          0,
+	Downloader:             "default",
 	DatabaseOpenAttempts:   -1,
 	Architectures:          []string{},
 	DepFollowSuggests:      false,

utils/config_test.go

@@ -63,6 +63,7 @@ func (s *ConfigSuite) TestSaveConfig(c *C) {
 		"  \"downloadConcurrency\": 5,\n"+
 		"  \"downloadSpeedLimit\": 0,\n"+
 		"  \"downloadRetries\": 0,\n"+
+		"  \"downloader\": \"\",\n"+
 		"  \"databaseOpenAttempts\": 5,\n"+
 		"  \"architectures\": null,\n"+
 		"  \"dependencyFollowSuggests\": false,\n"+