Allow to generate lower case header references through the config

This makes the references consistent/compatible with GitHub,
GitLab, Pandoc and many other tools.

This behavior can be enabled in config.json with:

```
"linkifyHeaderStyle": "gfm"
```

Signed-off-by: hoijui <hoijui.quaero@gmail.com>

config.json.example

@@ -123,6 +123,7 @@
         {
           "connectionString": "change this",
           "container": "change this"
-        }
+        },
+        "linkifyHeaderStyle": "gfm"
     }
 }