feat: add authorize callback for subscription requests

[melvincarvalho]

Summary

Add optional authorize callback that allows servers to check permissions before allowing WebSocket subscriptions.

Changes

• Add authorize(iri, req, callback) option called before each subscription
• If authorization fails, client receives err <url> forbidden
• Default allows all subscriptions (backwards compatible)
• Expose HTTP upgrade request on client for auth header/cookie access
• Update README with usage example

Usage

var solidWs = SolidWs(server, app, {
  authorize: function (iri, req, callback) {
    checkUserAccess(iri, req, function (err, hasAccess) {
      callback(err, hasAccess)
    })
  }
})

Fixes #28
Related: nodeSolidServer/node-solid-server#1334