Move settings/books to specific folder

[ImranR98]

By @ImranR98
It would be nice if we could store all book-related state (color scheme, font settings, annotations/highlights, current position in book, etc.) in a file in the same folder as the book.

Updated interpretation by @Acclorite
Store all settings/books in specific folder that user chooses. This will allow to sync folder between multiple devices with sync service like Syncthing and simplify #95.