Add a "--database <name> ..." flag to "bin/storage dump"
Summary: Ref T13671. Allow "bin/storage dump" to dump a subset of databases, primarily to support merging previously-partitioned databases.
Test Plan: Ran bin/storage dump with and without --database ... flags. Ran --database invalid, --database a --database a to hit error cases.
Maniphest Tasks: T13671
Differential Revision: https://secure.phabricator.com/D21745