Allow error reporting middleware to detect user

[shouichi]

Is your feature request related to a problem? Please describe.

Though there is no automatic way to set user field of error reporting, we usually rely on rack middleware to catch errors. It would be nice if the middleware automatically sets user from env.

Describe the solution you'd like

Set env["whatever"] allows the middleware to use that value as user.

Describe alternatives you've considered

Setting callback method on reporter.

config.on_report = -> (env, event) {
  event.user = env["whatever"]
}

Additional context

None.