Support exporting custom "Options/Select" fields to Excel/JSON/CSV/etc

Summary:
See https://discourse.phabricator-community.org/t/exporting-custom-select-fields-as-part-of-query-exports/2466/.

In JSON, export both the internal key and the visible value. For other formats, export the visible label.

Test Plan:

• Added a custom options/select field.
• Exported CSV, JSON, Text, got sensible output.

Reviewers: amckinley

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D20316