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>

lib/config/default.js

@@ -157,5 +157,6 @@ module.exports = {
   allowEmailRegister: true,
   allowGravatar: true,
   allowPDFExport: true,
-  openID: false
+  openID: false,
+  linkifyHeaderStyle: 'keep-case'
 }