Strict mode

[brettz9]

• Optimization: Use strict mode (avoiding strict-mode-invalid octal escape)
• Maintenance: Add .editorconfig
• CI: Add Node 12; drop Node 6 (won't work with eslint 6)
• npm: Indicate engines as a minimum of 8 (since only able to test >= 8 now when using eslint 6)
• npm: Update devDeps and dep. (entities--major bump but public-facing API to be same: https://github.com/fb55/entities/releases )

Fixes some vulnerabilities.

[rburns]

thanks for this!