Fix issue #674: parameter "flip" is added to [net] parameters for classifier and detector

flip=0 switches flip augmentation off.
Default is flip=1 which is consistent with a previous behaviour.

src/network.h

@@ -52,6 +52,7 @@ typedef struct network{
     int h, w, c;
     int max_crop;
     int min_crop;
+    int flip; // horizontal flip 50% probability augmentaiont for classifier training (default = 1)
     float angle;
     float aspect;
     float exposure;