Basic HTTP Header Viewer Script

Create a script that takes a URL and fetches its HTTP response headers. This is useful for quickly checking server details like `Server`, `Content-Type`, and `Set-Cookie`.  

### Expected Behavior  
- Input: single URL  
- Output: print all response headers in a clean format  
- Handle unreachable URLs gracefully  

### Example Usage  
```bash
python headers.py -u http://example.com
