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.

README.md

@@ -180,7 +180,15 @@ Example usage: **`--no-download`**
 Takes a file directory as a parameter and skips the posts if it matches with the post IDs inside the file. It also saves the newly downloaded posts to the given file.
 
 Example usage: **`--downloaded-posts D:\bdfr\ALL_POSTS.txt`**
-  
+
+## **`--config`**
+Specify a `config.json` file to use. This will disable reading from the default `Bulk downloader for reddit/config.json` file, as well as `--use-local-config` option.
+
+Example usage: **`--config /etc/bdfr/config.json`**
+Example usage: **`-c ~/config.json`**
+Example usage: **`--config c:\Users\Me\Downloads\config.json`**
+Example usage: **`-c c:\config.json`**
+
 ## ❔ FAQ
 
 ### I am running the script on a headless machine or on a remote server. How can I authenticate my reddit account?