kythe tool: by default, print --target_definitions if requested
kythe tool: by default, print --target_definitions if requested

Reviewers: #core_team, fromberger, shahms

Reviewed By: #core_team, fromberger, shahms

Subscribers: #core_team

Differential Revision: https://phabricator-dot-kythe-repo.appspot.com/D1719

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