Added a preUnsubscribe handler, analog to authorizeSubscribe

Invoked then a client unsubscribes from topics. Just like authorizeSubscribe, this function allows modifying the topics because executing the unsubscribe. This is needed in cases where topics are modified in authorizeSubscribe. For example:

```js
aedes.authorizeSubscribe = (client, subscription, callback) => {
  // overwrite subscription: force client to its namespace
  subscription.topic = `/${client.id}/${subscription.topic}`;
  callback(null, subscription);
}

aedes.preUnsubscribe = (client, packet) => {
  // overwrite unsubscriptions: force client to its namespace
  for (let i in packet.unsubscriptions) {
    packet.unsubscriptions[i] = `/${client.id}/${packet.unsubscriptions[i]}`;
  }
}
```

Includes fix-ups:
- Update docs/Aedes.md
  Co-authored-by: Daniel Lando <daniel.sorridi@gmail.com>
- added defaultPreUnsubscribe

aedes.js

@@ -25,6 +25,7 @@ const defaultOptions = {
   authorizePublish: defaultAuthorizePublish,
   authorizeSubscribe: defaultAuthorizeSubscribe,
   authorizeForward: defaultAuthorizeForward,
+  preUnsubscribe: defaultPreUnsubscribe,
   published: defaultPublished,
   trustProxy: false,
   trustedProxies: [],
@@ -69,6 +70,7 @@ function Aedes (opts) {
   this.authorizePublish = opts.authorizePublish
   this.authorizeSubscribe = opts.authorizeSubscribe
   this.authorizeForward = opts.authorizeForward
+  this.preUnsubscribe = opts.preUnsubscribe
   this.published = opts.published
 
   this.decodeProtocol = opts.decodeProtocol
@@ -337,6 +339,8 @@ function defaultAuthorizeForward (client, packet) {
   return packet
 }
 
+function defaultPreUnsubscribe (client, packet) { }
+
 function defaultPublished (packet, client, callback) {
   callback(null)
 }

docs/Aedes.md

@@ -29,6 +29,7 @@
   - [Handler: authorizePublish (client, packet, callback)](#handler-authorizepublish-client-packet-callback)
   - [Handler: authorizeSubscribe (client, subscription, callback)](#handler-authorizesubscribe-client-subscription-callback)
   - [Handler: authorizeForward (client, packet)](#handler-authorizeforward-client-packet)
+  - [Handler: preUnsubscribe (client, packet)](#handler-preunsubscribe-client-packet)
   - [Handler: published (packet, client, callback)](#handler-published-packet-client-callback)
 
 ## new Aedes([options]) / new Aedes.Server([options])
@@ -386,6 +387,28 @@ aedes.authorizeForward = function (client, packet) {
 }
 ```
 
+## Handler: preUnsubscribe (client, packet)
+
+- client: [`<Client>`](./Client.md)
+- packet: `<aedes-packet>` & [`UNSUBSCRIBE`][UNSUBSCRIBE]
+
+Invoked when a client unsubscribes from topics. Just like `authorizeSubscribe`, this function allows modifying the topics before executing the unsubscribe. This may be needed in cases where topics were modified in authorizeSubscribe. For example:
+
+```js
+aedes.authorizeSubscribe = (client, subscription, callback) => {
+  // overwrite subscription: force client to its namespace
+  subscription.topic = `/${client.id}/${subscription.topic}`;
+  callback(null, subscription);
+}
+
+aedes.preUnsubscribe = (client, packet) => {
+  // overwrite unsubscriptions: force client to its namespace
+  for (let i in packet.unsubscriptions) {
+    packet.unsubscriptions[i] = `/${client.id}/${packet.unsubscriptions[i]}`;
+  }
+}
+```
+
 ## Handler: published (packet, client, callback)
 
 - packet: `<aedes-packet>` & [`PUBLISH`][PUBLISH]
@@ -397,6 +420,7 @@ same as [`Event: publish`](#event-publish), but provides a backpressure function
 [CONNECT]: https://github.com/mqttjs/mqtt-packet#connect
 [CONNACK]: https://github.com/mqttjs/mqtt-packet#connack
 [SUBSCRIBE]: https://github.com/mqttjs/mqtt-packet#subscribe
+[UNSUBSCRIBE]: https://github.com/mqttjs/mqtt-packet#unsubscribe
 [PINGREQ]: https://github.com/mqttjs/mqtt-packet#pingreq
 [PUBLISH]: https://github.com/mqttjs/mqtt-packet#publish
 [PUBREL]: https://github.com/mqttjs/mqtt-packet#pubrel

lib/handlers/unsubscribe.js

@@ -16,6 +16,10 @@ function UnsubscribeState (client, packet, finish) {
 
 function handleUnsubscribe (client, packet, done) {
   const broker = client.broker
+  if (broker.preUnsubscribe) {
+    broker.preUnsubscribe(client, packet)
+  }
+
   const unsubscriptions = packet.unsubscriptions
   let err