fix: missing implementation of --exclude

[zostay]

Fixes #42

This does the following:

• The --exclude option is now implemented. Previously, the flag was defined, but was ignored entirely.
• Exclusions set by either --exclude-index or --exclude will also impact random when the --book option is used. Previously, the --book option ignored exclusions.

This pull request includes changes to improve the handling of excluded references and enhance error handling when looking up books in the random package. The most important changes include initializing the excludeRefs slice, appending to excludeRefs correctly, and improving error messages for book lookups.

Improvements to handling excluded references:

• cmd/random.go: Initialized the excludeRefs slice and appended excluded references to it before applying the ExcludeReferences option. [1] [2]

Enhancements to error handling:

• pkg/ref/random.go: Improved error handling when looking up books by adding detailed error messages and ensuring the first verse of the book is used correctly.