Feature request: fetch individual item history

[shamblesides]

Proposed usage:

userbase.itemHistory({ databaseName, itemId }).then(revisions => {
  for (const revision of revisions) {
    const { item, username, userDeleted, timestamp } = revision
    // maybe file stuff too? Not sure if old files are stored permanently
  }
})

Maybe would always be fetched from transaction log rather than the bundle.