add a DebugInfo switch

Signed-off-by: pengzhile <pengzhile@gmail.com>

README.md

@@ -49,4 +49,5 @@ EQUAL,somedomain
 ## Debug info
 
 * the `ja-netfilter` will **NOT** output debugging information by default
-* add environment variable `JANF_DEBUG=1` and start to enable it
+* add environment variable `JANF_DEBUG=1` and start to enable it
+* or add system property `-Djanf.debug=1` to enable it

src/main/java/io/zhile/research/ja/netfilter/commons/DebugInfo.java

@@ -3,7 +3,7 @@ package io.zhile.research.ja.netfilter.commons;
 import io.zhile.research.ja.netfilter.utils.DateUtils;
 
 public class DebugInfo {
-    private static final boolean DEBUG = "1".equals(System.getenv("JANF_DEBUG"));
+    private static final boolean DEBUG = "1".equals(System.getenv("JANF_DEBUG")) || "1".equals(System.getProperty("janf.debug"));
 
     public static void output(String content) { // No logger lib required
         if (!DEBUG) {