Show tiles from both offline and cached provider #631

First, thanks to @johnpryan for a great package.

I have created png tiles zoom level 1-6 for the entire planet and using them as assets with AssetTileProvider. I will consider packaging the png files and then unpacking into the file system to use FileTileProvider because it seems I have to enter every subdirectory separately in pubspec.yaml and that seems to slow down hot reloads (if anyone has a better suggestions, please...)

Anyway, the zoom 1-6 is good enough for the offline use, but when the user is online I of course want to be able to zoom in more using a normal CachedNetworkImageProvider so I choose provider depending on whether the user is online or offline.

Now, what I really want is to enable the offline user to zoom in more and show the tiles from the CachedNetworkImageProvider if those tiles are in the cache (if not in the cache perhaps show a default png saying something like "NOT AVAILABLE"). Is there any way of doing that already or would that require a new Provider?