Is your feature request related to a problem? Please describe.
Would be great to be able to open book at some previously identified position. It would be useful when making annotations on book in note-taking SW like Obsidian etc. This could provide ability to proceed to related book position or fragment by following link in a note.

Describe the solution you'd like
Basically I'd propose to consider two improvements:

1. support EPUB CFI in command line arguments specifying file to open, so the user is able to launch Foliate like com.github.johnfactotum.Foliate 'file:///home/user/Downloads/Alices\ Adventures\ in\ Wonderland.epub#epubcfi(/6/20!/4/2/2/1:0)' or so (maybe worth support escaping/unescaping CFI syntax).
2. support custom URI scheme denoting book in library. This could be epub, ebook or anything. Specific book in "path" part of URI could be represented by UUID or any supported ID. Examples:

• epub:eb2934ae-bb1a-4652-bce7-9f78fc5ca496 for book itself
• epub:eb2934ae-bb1a-4652-bce7-9f78fc5ca496#epubcfi(/6/20!/4/2/2/1:0) for position in book

Foliate could be associated with such scheme at system level. Using such custom scheme would allow keep links in notes alive among devices provided that user syncs also his library.

Besides position specifying fragment would be also handy. In that case I'd propose have chosen text fragment being selected at start of application.