Ability to hide folders on predicate.

[Antoshjkee]

Based on my question #111
Since change is so small, I have implemented the change before getting a reply.

Overall:
The current feature allows hiding folders on a predicate during the config setup.

Example:

    treeConfig.AddFolder("Folder", folderConfig =>
    {
        folderConfig.HideFromTree(() =>
        {
            var currentUser = UmbracoContext.Current.Security.CurrentUser;
            if (currentUser == null) return false;
            return currentUser.IsAdmin();
        });

        // ... 
    }    

[mattbrailsford]

Hey @Antoshjkee, thanks for submitting a PR. I'm currently away ATM, back next week, so will review properly then, but initial thought on the idea is that it's s good one.

Will come back with more in a few days.

Many thanks