FR: Support using directory for "--output"

[mike-pt]

This would be useful when used with download mode and continue.

Since continue forces us to pass the output file it means the file name would need to be static, but in some cases (Scripts) we might need to still get the file name from the server (or guess it) and continue if a partial file already exists.

In cases where the server provided Content-Disposition what I do now is get that then use it for the "-o" filename, but when this is not an option things get more complicated so having this would help.

[akshgpt7]

Hey, I'd give this a shot.