Option for forcing the inclusion of the hour field in WebVTT timestamps

[ejm]

While the ​WebVTT spec denotes that the hour field is optional on timestamps whose duration is less than one hour, it would be helpful to be able to forcibly include the field, even if the value is 00, with an option.

Currently, this field is set in ​webvtt_write_time by checking if hour > 0.

The usecase here is with a third-party tool that expects the hour field regardless of its content, despite the spec, so I understand if that is too niche to justify inclusion.

Thank you!