feature request: block schema

similar to direct but block

```
pproxy \
  -r direct://?direct.rules \      # Bypass proxy for domains in direct.rules
  -r block://?block.rules \        # Block domains in block.rules
  -r socks5://127.0.0.1:1080 \     # Default: Proxy everything else
  -l http://0.0.0.0:3080
```