Add the ability to download files from pastes.dev and bytebin

[funnyboy-roks]

If --download <url> is passed and a file is given, it will overwrite the contents of the file. If file is not given, then it should download to the <key>.extension. I think we can use the mime2ext crate to guess the extension.

The url should be allowed to be either the "gui url" or "api url" (See UrlSet)

In addition to allowing the url, I'd like to have it such that if --download <key> and (--bytebin or --pastes) are passed, then it will download using that key from bytebin or pastes.dev.

If you desire to complete this, please reply to this issue so we can discuss it further.