new option: weasyprint_log_path

[michalfapso]

For setting an output path for the weasyprint log for easier debugging of weasyprint issues.

Although there was already the following code in plugin.py:

from weasyprint.logger import LOGGER
if self._options.verbose:
    LOGGER.setLevel(logging.DEBUG)
    self._logger.setLevel(logging.DEBUG)
else:
    LOGGER.setLevel(logging.ERROR)

If I understand it correctly, this is supposed to show the weasyprint log in the mkdocs output when verbose: true, but I don't see it there. That's why I made this new option for writing it to a separate file.