Add a feature to normalize book entries

**Is your feature request related to a problem? Please describe.**
There is often a mismatch with the capitalization of book entries. For example sometimes all letters are in lowercase, some are uppercase and so on. This might distract the reader of a book list. It would be nice to have a method to automatically transform the texts and normalize them in a way.

**Describe the solution you'd like**
A new subcommand `modify` or `transform` might be implemented. In a first step it might be sufficient to add options like `--title-uppercase` and similar. Another step could be to introduce templates for this.

Start by locating the existing CLI subcommand structure and the code that handles book entries. Clarify whether the first deliverable is a `modify` or `transform` subcommand with options such as `--title-uppercase`, and define the normalization rules before implementation. Done means the chosen options consistently transform book-entry text and are covered by appropriate tests.

Written by the indexing model from the issue text.