ConfigParser: do NOT override existing sections

This commit support format as below:

```
[DNS]
EQUAL,example.com

[URL]
PREFIX,https://example.com/a

[URL]
PREFIX,https://example.com/foo

[URL]
PREFIX,https://example.com/bar
```

It merges sections with the same section name.

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

@@ -45,7 +45,10 @@ public class ConfigParser {
                         }
 
                         lastSection = section;
-                        map.put(lastSection, new ArrayList<>());
+                        if (null == map.get(lastSection)) {
+                            // do NOT override existing sections
+                            map.put(lastSection, new ArrayList<>());
+                        }
                         break;
                     case '#':
                     case ';':