Support Custom Cover Pages

[haroldtreen]

Summary

@wrobbins recently added support for user provided cover images:

https://github.com/haroldtreen/epub-press/pull/34

The clients however are still very specific about what they send to the backend:

https://github.com/haroldtreen/epub-press-clients/blob/master/packages/epub-press-js/epub-press.js#L37-L54

In order for users to take advantage of the backend feature we will need to update the clients.

Acceptance Criteria

• [ ] epub-press-js can accept the additional metadata
• [ ] epub-press-js has docs for these new params (added here )
• [ ] Bonus: A field is added in epub-press-chrome for the extension.
  • Additional fields should be hidden by default and only shown if expanded? Maybe we save if the user has expanded these extra fields and consistently show the fields between usages.
  • This is a stretch - another issue to followup on this would be sufficient.