title: Leaflet tile provider with fallback and grey scale
body: <p>I'm currently using two plugins for leaflet:
  <a href="https://github.com/ghybs/Leaflet.TileLayer.Fallback" rel="nofollow noreferrer">https://github.com/ghybs/Leaflet.TileLayer.Fallback</a> to allow my map to fall back to higher zoom levels if a tile for the current zoom level is not available.
  <a href="https://github.com/Zverik/leaflet-grayscale" rel="nofollow noreferrer">https://github.com/Zverik/leaflet-grayscale</a> to make my background map tiles grey scale.
  I would like to be able to do both at the same time.</p>
  
  <p>I tried making the grey scale plugin extend the fallback object, but that didn't work.</p>
  
  <p>How can I make leaflet-greyscale rather extend TileLayer.Fallback rather than just TileLayer?</p>
  
  <p>Ideally I want a new FallbackGreyscale object or maybe something like L.tileLayer.fallback.greyscale. But any way in which I can have both fallback and grey scale at the same time would be nice.</p>