Using '-' as filename for --export-* should not print intermediate results

[David Peter]

When using - to export to standard out, we get intermediate outputs that clutter the output.

How to reproduce?

hyperfine --runs 2 --export-markdown - 'sleep 0.1' 'sleep 0.2'

Expected result: the exported content should only be shown once at the end. Ideally, after the Summary section.

FYI @humblepenguinn