Additional changes:
- Allow empty value for PIA region
- Most settings are lowercased
- `OPENVPN_VERBOSITY` environment variable
- openvpn also tunnels IPv6, and unbound supports ipv6
- auth kept only on disk, not in memory
- readme reworked
- CI script fixed and improved
- Added v2 Docker tag
- Shadowsocks log defaults to `off`
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