Add --config option for explicitly stating configuration file to use.

aliparlakci/bulk-downloader-for-reddit#122

Address comments in pull review.

* Removed vertical space.
* Added config file existence check with log message.

src/arguments.py

@@ -153,9 +153,15 @@ class Arguments:
                             action="store_true",
                             help="Just saved posts into a the POSTS.json file without downloading"
                             )
-   
+
+        parser.add_argument("--config","-c",
+                            help="Specify exact config.json file to use. " \
+                            "Disables reading from 'Bulk downloader for " \
+                            "reddit/config.json' and --use-local-config " \
+                            "option."
+                            )
 
         if arguments == []:
             return parser.parse_args()
         else:
-            return parser.parse_args(arguments)
+            return parser.parse_args(arguments)