Support `-` as shorthand for stdout for output

[horenmar]

Related to my other issue, I tried using --export-markdown - to get the markdown output shown immediately in stdout. To my surprise, I ended up with a file named -.

Since using - for stdout/stderr is relatively common convention, I would like it to be supported for the output options as well.

[sharkdp]

I use --export-markdown /dev/stdout, but supporting - sounds good. Thank you for the suggestion.