R1:917fedafe69e

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.