Implement something like `drop`

This is a reminder to myself to implement a function I mentioned on Stack Overflow:

http://stackoverflow.com/questions/28421469/how-to-drop-lines-when-streaming-from-a-file-using-haskell-and-the-turtle-librar/28422201#28422201

Here is the implementation:

``` haskell
import Data.IORef
import Turtle

drop :: Int -> Shell a -> Shell a
drop n s = Shell (\(FoldM step begin done) -> do
ref <- newIORef 0
let step' x a = do
n' <- readIORef ref
writeIORef ref (n' + 1)
if n' < n then return x else step x a
foldIO s (FoldM step' begin done) )
```