Epic : missing options for creation/update

Created by: nicolasbrechet

Hi,

In the CreateEpicOptions struct, we're missing some options, which in my case prevents me from creating a "child" Epic https://github.com/xanzy/go-gitlab/blob/1fb53be1c6e8a7dd9dd1b2c4520df8489b4c7355/epics.go#L176

• color
• confidential
• created_at
• parent_id

And as well in UpdateEpicOptions https://github.com/xanzy/go-gitlab/blob/1fb53be1c6e8a7dd9dd1b2c4520df8489b4c7355/epics.go#L213

• add_labels
• parent_id
• remove_labels
• updated_at
• color

I'm working on a PR to fix this