Add userContext

[ai]

We may have some context of the user, like language. It should be synchronized with connect message.

[polemius]

I would like to implement this feature. Could you please provide more details?

[ai]

This issue was created to solve question number 1 from this question https://gitter.im/logux/logux?at=5e93679e3a85536c4325e719

We need to add options like:

new Client({
  userId,
  token,
  userContext: {
    locale
  }
})

To start this issue, you need to suggest protocol and API changes.