Add a option SDWebImageFromCacheOnly to load the image from cache only and prevent network

@@ -98,15 +98,20 @@ typedef NS_OPTIONS(NSUInteger, SDWebImageOptions) {
     
     /**
      * By default, we do not query disk data when the image is cached in memory. This mask can force to query disk data at the same time.
-     * This options is recommend to be used with `SDWebImageQueryDiskSync` to ensure the image is loaded in the same runloop.
+     * This flag is recommend to be used with `SDWebImageQueryDiskSync` to ensure the image is loaded in the same runloop.
      */
     SDWebImageQueryDataWhenInMemory = 1 << 13,
     
     /**
      * By default, we query the memory cache synchronously, disk cache asynchronously. This mask can force to query disk cache synchronously to ensure that image is loaded in the same runloop.
-     * This can avoid flashing during cell reuse if you disable memory cache or in some other cases.
+     * This flag can avoid flashing during cell reuse if you disable memory cache or in some other cases.
      */
-    SDWebImageQueryDiskSync = 1 << 14
+    SDWebImageQueryDiskSync = 1 << 14,
+    
+    /**
+     * By default, when the cache missed, the image is download from the network. This flag can prevent network to load from cache only.
+     */
+    SDWebImageFromCacheOnly = 1 << 15
 };
 
 typedef void(^SDExternalCompletionBlock)(UIImage * _Nullable image, NSError * _Nullable error, SDImageCacheType cacheType, NSURL * _Nullable imageURL);

@@ -157,8 +157,12 @@
             [self safelyRemoveOperationFromRunning:strongOperation];
             return;
         }
-
-        if ((!cachedImage || options & SDWebImageRefreshCached) && (![self.delegate respondsToSelector:@selector(imageManager:shouldDownloadImageForURL:)] || [self.delegate imageManager:self shouldDownloadImageForURL:url])) {
+        
+        // Check whether we should download image from network
+        BOOL shouldDownload = (!(options & SDWebImageFromCacheOnly))
+            && (!cachedImage || options & SDWebImageRefreshCached)
+            && (![self.delegate respondsToSelector:@selector(imageManager:shouldDownloadImageForURL:)] || [self.delegate imageManager:self shouldDownloadImageForURL:url]);
+        if (shouldDownload) {
             if (cachedImage && options & SDWebImageRefreshCached) {
                 // If image was found in the cache but SDWebImageRefreshCached is provided, notify about the cached image
                 // AND try to re-download it in order to let a chance to NSURLCache to refresh it from server.