R1:a79bb55f3f6b

Support CSV, JSON, and tab-separated text as export formats

Summary: Depends on D18919. Ref T13046. Adds some simple modular exporters.

Test Plan: Exported pull logs in each format.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13046

Differential Revision: https://secure.phabricator.com/D18934