In "bin/bulk export", require "--output <path>" by default

Summary:
Depends on D19743. Ref T13210. Since this command can easily dump a bunch of binary data (or just a huge long blob of nonsense) to stdout, default to requiring "--output <file>".

Using --output - will print to stdout.

Test Plan: Ran with: no --output, --output file, --output -, --output - --overwrite. Got sensible results or errors in all cases.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13210

Differential Revision: https://secure.phabricator.com/D19744