Handle UTF-8 output for reports

[mmaunder]

Fixes #327.

Summary

• open report output files with an explicit UTF-8 encoding to support non-ASCII characters
• add a regression test that verifies the UTF-8 encoding flag is passed to the file handle

Testing

• ./venv/bin/python -m unittest discover -s wordfence -t .
• ./venv/bin/python -m flake8 --exclude venv --require-plugins pycodestyle,flake8-bugbear