Optimize network traffic (use cache first if requested)

This is a proof-of-concept of how to implement it.
Tests are needed.
Also, I believe there's an overlap with fileCache, which needs some figuring out.

src/android/src/main/java/com/RNFetchBlob/RNFetchBlobConfig.java

     public String appendExt;
     public ReadableMap addAndroidDownloads;
     public Boolean trusty;
+    public String key;
 
     RNFetchBlobConfig(ReadableMap options) {
         if(options == null)
         if(options.hasKey("addAndroidDownloads")) {
             this.addAndroidDownloads = options.getMap("addAndroidDownloads");
         }
+        this.key = options.hasKey("key") ? options.getString("key") : null;
     }
 
 }

src/android/src/main/java/com/RNFetchBlob/RNFetchBlobFileHandler.java

     String mTaskId;
     RNFetchBlobConfig mConfig;
 
-    RNFetchBlobFileHandler(ReactApplicationContext ctx, String taskId, RNFetchBlobConfig config, Callback onResponse) {
-        super(new File( RNFetchBlobFileHandler.getFilePath(ctx, taskId, config)), false, false);
+    RNFetchBlobFileHandler(ReactApplicationContext ctx, String taskId, String key, RNFetchBlobConfig config, Callback onResponse) {
+        super(new File( RNFetchBlobFileHandler.getFilePath(ctx, taskId, key, config)), false, false);
         this.onResponse = onResponse;
         this.mTaskId = taskId;
         this.mConfig = config;
         this.mCtx = ctx;
-        if(!new File(RNFetchBlobFileHandler.getFilePath(ctx, taskId, config)).isFile()) {
+        if(!new File(RNFetchBlobFileHandler.getFilePath(ctx, taskId, key, config)).isFile()) {
             this.isValid = false;
         }
         this.isValid = true;
     }
 
-    static String getFilePath(ReactApplicationContext ctx, String taskId, RNFetchBlobConfig config) {
+    static String getFilePath(ReactApplicationContext ctx, String taskId, String key, RNFetchBlobConfig config) {
         if(config.path != null)
             return config.path;
         else if(config.fileCache && config.appendExt != null)
-            return RNFetchBlobFS.getTmpPath(ctx, taskId) + "." + config.appendExt;
+            return RNFetchBlobFS.getTmpPath(ctx, key) + "." + config.appendExt;
         else
-            return RNFetchBlobFS.getTmpPath(ctx, taskId);
+            return RNFetchBlobFS.getTmpPath(ctx, key);
     }
 
     @Override

src/android/src/main/java/com/RNFetchBlob/RNFetchBlobReq.java

 
 import java.io.File;
 import java.security.KeyStore;
+import java.security.MessageDigest;
 
 import cz.msebera.android.httpclient.HttpEntity;
 import cz.msebera.android.httpclient.entity.ByteArrayEntity;
         }
     }
 
+    public static String getMD5(String input) {
+        MessageDigest md = MessageDigest.getInstance("MD5");
+        md.update(input.getBytes());
+        byte[] digest = md.digest();
+        
+        StringBuffer sb = new StringBuffer();
+        
+        for (byte b : digest) {
+            sb.append(String.format("%02x", b & 0xff))
+        }
+
+        return sb.toString();
+    }
+
     @Override
     public void run() {
 
 
         }
 
+        String key = this.taskId;
+        if (this.options.key != null) {
+            key = RNFetchBlobReq.getMD5(this.options.key);
+
+            File file = new File(RNFetchBlobFileHandler.getFilePath(ctx, taskId, key, this.options))
+            if (file.exists()) {
+               callback.invoke(null, file.getAbsolutePath());
+               return;
+            }
+        }
+
         try {
 
             req = new AsyncHttpClient();
 
             // create handler
             if(options.fileCache || options.path != null) {
-                handler = new RNFetchBlobFileHandler(ctx, taskId, options, callback);
+                handler = new RNFetchBlobFileHandler(ctx, taskId, key, options, callback);
                 // if path format invalid, throw error
                 if (!((RNFetchBlobFileHandler)handler).isValid) {
                     callback.invoke("RNFetchBlob fetch error, configuration path `"+ options.path  +"` is not a valid path.");

src/index.js

  *                   If this property has a valid string format, resonse data
  *                   will be saved to specific file path. Default string format
  *                   is : `RNFetchBlob-file://path-to-file`
+ *         @property {string} key
+ *                   If this property is set, it will be converted to md5, to
+ *                   check if a file with this name exists.
+ *                   If it exists, the absolute path is returned (no network 
+ *                   activity takes place )
+ *                   If it doesn't exist, the file is downloaded as usual
  *
  * @return {function} This method returns a `fetch` method instance.
  */
       else {
         let respType = 'base64'
         // response data is saved to storage
-        if(options.path || options.fileCache || options.addAndroidDownloads) {
+        if(options.path || options.fileCache || options.addAndroidDownloads || options.key) {
           respType = 'path'
           if(options.session)
             session(options.session).add(data)