Closest command is currently not implemented

[heuermh]

https://github.com/bigdatagenomics/lime/blob/master/lime-cli/src/main/scala/org/bdgenomics/lime/cli/Closest.scala#L56

[devin-petersohn]

The challenge with this is that with the updated implementation, it's not possible to do this without triggering a full crossproduct if you don't bound the closest operation.