AddSection title cannot be empty

[bmaupin]

As per #21, an empty title is invalid.

However, that's currently used as the mechanism to add a section to an EPUB without adding it to the table of contents:

The title is optional; if no title is provided, the section will not be added to the table of contents.

https://pkg.go.dev/github.com/bmaupin/go-epub?utm_source=godoc#Epub.AddSection

Two problems need to be solved:

1. What should the appropriate behaviour be when an empty title is added?

   This seems to be related to the philosophy of how this library should handle errors... Should we

   1. Throw an error if an empty title is added?
   2. Generate a title if an empty title is added?
   3. Keep the current behaviour but make a note in the documentation that an empty title is invalid?

2. How should we handle the ability to add a section to the EPUB without adding it to the table of contents? Some possibilities:

   • Keep the current behaviour (conflicts with Add feature: Table of Contents support unlimited levels #17)
   • Remove this functionality
   • Add this functionality in a different way