Give data exporters a header row

Summary:
Depends on D18951. Ref T13049. When we export to CSV or plain text, add a header row in the first line of the file to explain what each column means. This often isn't obvious with PHIDs, etc.

JSON has keys and is essentially self-labeling, so don't do anything special.

Test Plan: Exported CSV and text, saw new headers. Exported JSON, no changes.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13049

Differential Revision: https://secure.phabricator.com/D18952