Implemented support for different export formats.

Summary:
This adds support for different export formats to Excel
via a drop-down on the Export page as per the discussion
in D5443.

Test Plan:
Export some things from Maniphest. Do a simple implementation
of ManiphestExcelFormat just for testing and make sure that
it appears in the list after you run arc liberate.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T2575

Differential Revision: https://secure.phabricator.com/D5642

Conflicts:

src/applications/maniphest/controller/ManiphestExportController.php