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

[akenion]

I'm not sure simply switching the report encoding to UTF-8 resolves this. The matched string from scanned files is included, scanned files aren't guaranteed to be UTF-8, and not all binary sequences are valid UTF-8, so I think we need to explore a different solution here.