Summary:
- like other subcommands, reporting compression sizes should be specified with the `--command` CLI arg.
- also added `--compression_types` arg as it's useful to restrict the types of compression used, at least in my dictionary compression experiments.
Closes https://github.com/facebook/rocksdb/pull/2706

Differential Revision: D5589520

Pulled By: ajkr

fbshipit-source-id: 305bb4ebcc95eecc8a85523cd3b1050619c9ddc5