Option to use relative path when save_per_work_dir=1

[fenghse]

Add an option to use the relative path instead of abs path. Thus the .vimbook file can be portable with the folder.

[arooni]

Ditto! This would be awesome!

I work on the same checkout of files but on different machines so it seems this suggestion would be the only way to get my bookmarks to synchronize across machines

[MattesGroeger]

Ah thx for the feature request! That's a good point if you want to sync the bookmark files between different machines. It will be a bit tricky to introduce without breaking changes, though.

[TryerGit]

If this feature were implemented, it would go a long way in enabling seamless cross-platform development. It would surely be a great productivity boost! I referred to this thread here: https://vi.stackexchange.com/questions/34362/managing-multifile-projects-using-a-portable-bookmark-plugin