support SliceRange in deletion

I'd like to be able to delete a range of columns within a row, for a range of column names. Currently it seems my only option is to do a range query and then individually delete each column. For wide rows, this is very slow, even with batch mutation.

The thrift API looks like it supports the call, however it throws the following error message: "Deletion does not yet
support SliceRange predicates.". I'm talking specifically about batch_mutate + Mutation(Deletion(SlicePredicate(SliceRange(start, end))).