enhance proxy detection.

User can choose which header will be used for guessing a real ip.
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Merge pull request #15 from Kennyl/ipdection

- CHG: 新增 $PROXYHEADERlist 設定，在TRUST_HTTP_X_FORWARDED_FOR啟用的情況下可以支援更多Proxy取得真實IP (檔頭容易有偽造風險，請謹慎評估) (Thanks Kenny)