Add playerView option to plugin API

[delucis]

Currently games can define a playerView to exclude secret state from the G that is returned to clients.

As mentioned in #668, plugins should also be able to hide secret state on the client, for example using a playerView option:

const plugin = {
  playerView: ({ G, ctx, game, data, playerID }) => data without secrets,
}

We would then have to call the plugins’ playerView for each update sent to clients.