Add 'Subtitle' as a new field for books

[aethior]

Some books have a short title followed by a subtitle. In book apps, the full title (title + subtitle) is usually included to ensure that no information is omitted, since there isn’t a specific field for the subtitle. I think it would be a useful feature to include in this app.

Example:

Title: The Corrosion of Character
Subtitle: The Personal Consequences of Work in the New Capitalism

Better and cleaner than 'The Corrosion of Character: The Personal Consequences of Work in the New Capitalism'.

[Mozzo1000]

This has been added with commit 2a0f332
If a book has a subtitle it will display it underneath the title on the books page. If a book does not have a subtitle nothing will render.

The subtitle is not present in search results.

Thank you @aethior for this feature request, it will be included in the next release.