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