Add option to exclude subsites from displaying the message

Adds an option to exclude subsites (objects providing `INavigationRoot`) from displaying the message.
I've taken the exclusion approach because it causes new subsites to be included by default.

When having nested subsites, the nearest parent subsite is checked for whether it is included or excluded.

@hvelarde this should solve #12 , can you take a look at it and test whether it is working how you would expect it?

**TODO**

- [x] Update screenshot in readme
- [x] Update readme, describe the feature
- [x] Verify: deleting a excluded subsite should not break vocabulary constraints

// @maethu 
Closes #12