feat: optional stdout decoding

[Sofurs]

Please allow us to convert text (e.g. markdown) to PDF format using convert_text function with the outputfile parameter set to "-" (stdout).

try:
    stdout = stdout.decode('utf-8')
except UnicodeDecodeError:
    # this shouldn't happen: pandoc more or less guarantees that the output is utf-8!
    raise RuntimeError('Pandoc output was not utf-8.')

Currently the function raises the UnicodeDecodeError exception.