Add STDOUT output to mutool-convert

[arbitrary-dev]

Because it's not good to mislead a user in a manual.

[haliumsolutions]

also -o always puts page number in filename regardless of '%d' or not

[Maskedman99]

@robinwatts mutool draw is not outputting to stdout even though the document claims it to do so (Doc). It's reported at Bugzilla.

$ mutool draw -F png 9781586380199.epub 1 | chafa 
chafa: Failed to open '-': Could not cache input stream