ensure that the config directory exists

lib/config.go

@@ -13,11 +13,15 @@ type FileConfig struct {
 }
 
 func NewFileConfig(rootDir string) *FileConfig {
-	os.Mkdir(rootDir, 0700)
 	return &FileConfig{rootDir: rootDir}
 }
 
 func (fc *FileConfig) Set(key string, value string) error {
+
+	// Ignore errors if the directory already exists.
+	os.Mkdir(fc.rootDir, 0700)
+
+	// Dump the value to file.
 	data := fmt.Sprintf("%s\n", value)
 	return ioutil.WriteFile(
 		filepath.Join(fc.rootDir, key),