added configuration validation and reset

Usage.md

@@ -63,6 +63,9 @@ Example that performs single detection and then draws new interpolated result at
                               // if image is not provided, it will generate internal sample
   human.models        // dynamically maintained list of object of any loaded models
   human.tf            // instance of tfjs used by human
+  human.reset()       // reset current configuration to default values
+  human.validate(config?) // validate configuration values
+                      // if config is not provided it will validate current values
 ```
 
 ## Face Recognition