Add multiple media support & ask credentials during 1st run

[nitinkumar30]

1. In this script, you can just save 1 media at a time & it'll be saved as secret.jpg or secret.mp4 . I want to make sure it can save multiple media files once ran, like USERNAME_TIMESTAMP.jpg or .mp4

2. We can also enhance it by asking users about API_KEY & API_HASH during 1st run so that we don't need to modify anything else to run it. It can handle things after that.