Support for `custom_fields` endoint

[bk-one]

Do you plan to support the https://harvest.greenhouse.io/v1/custom_field endpoint in the future? Or if not, would a PR have any chance of being accepted if it's in the desired quality? Just wondering if I need to implement support independently of this gem or if there's a chance that I can query details about a custom fields in the future using the gem.

Cheers
Ben