BREAKING CHANGE: v0.16 on('friend`) arguments changed!

[huan]

FriendRequest class refactored.

Before (v0.14 or below)

wechaty.on('friend', (contact, request) => {
  if (!request) {
    // this is a friend request confirmation event
  } else {
    // this is a friend request
  }
})

After (v0.16 or above)

wechaty.on('friendship', request => {
  switch (request.type()) {
    case FriendRequest.Type.RECEIVE:
      // this is a friend request request
      break
    case FriendRequest.Type.CONFIRM:
      // this is a friend request confirmation
      break
  }
})