title: How to extend a "SpatialPolygonsDataFrame" to a given extent
body: <p>I have a shapefile which I would like to extend (basically adding some NA rows and cols). Is there any function which does that like <code>raster::extend</code>? Unfortunately this only works for <code>raster</code> objects.</p>
  
  <p>I can crop it using <code>raster::crop</code>, which has a method for <code>SpatialPolygonsDataFrame</code>, but I am not able to find anything for extending.</p>