Version 2.6

New API:

• Paper.bookOn(path) to set custom storage location;
• book.getPath() or book.getPath(key) to get path for content of book or key.

Improvements:

• simultaneous read/write for different keys, up to 97% performance gain per thread.
• name change: use book.contains(key) instead of deprecated book.exist(key)

Thanks @hiperioncn and @cezar-carneiro for your contribution!