[FEATURE] Added 'autosizeContent' to FRLayeredNavigationItem

This pull request adds `FRLayeredNavigationItem.autosizeContent`. This allows you to size layers differently from their content controller view, giving you the ability to do things like add decorations or other visual niceties which may not be available in your controller (or take too much custom work), such as adding images on the right border to a layer containing a `UITableViewController`.

The default value is consistent with the current behavior.