Adding an include template for extra dev tools to be added.

I have added an include template to allow for extra dev tools to be added by a user without needing to override the `BetterNavigator.ss` template.

Currently if a user wants to add extra tools they will need to over the whole `BetterNavigator.ss`. It would be nice to get updates to the existing template while also being able to add addition tools. 

By including a separate template this allows the user to override just the `BetterNavigatorExtraDevTools.ss` template to add extra tools.