feat: allow transport options to be passed on creation (#524)

* feat: allow transport options to be passed on creation

* fix: only add circuit transport if enabled

* chore: fix lint

doc/CONFIGURATION.md

@@ -23,6 +23,7 @@
       - [Configuring Dialing](#configuring-dialing)
       - [Configuring Connection Manager](#configuring-connection-manager)
       - [Configuring Metrics](#configuring-metrics)
+      - [Customizing Transports](#customizing-transports)
   - [Configuration examples](#configuration-examples)
 
 ## Overview
@@ -499,6 +500,34 @@ const node = await Libp2p.create({
 })
 ```
 
+#### Customizing Transports
+
+Some Transports can be passed additional options when they are created. For example, `libp2p-webrtc-star` accepts an optional, custom `wrtc` implementation. In addition to libp2p passing itself and an `Upgrader` to handle connection upgrading, libp2p will also pass the options, if they are provided, from `config.transport`.
+
+```js
+const Libp2p = require('libp2p')
+const WebRTCStar = require('libp2p-webrtc-star')
+const MPLEX = require('libp2p-mplex')
+const SECIO = require('libp2p-secio')
+const wrtc = require('wrtc')
+
+const transportKey = WebRTCStar.prototype[Symbol.toStringTag]
+const node = await Libp2p.create({
+  modules: {
+    transport: [WebRTCStar],
+    streamMuxer: [MPLEX],
+    connEncryption: [SECIO]
+  },
+  config: {
+    transport: {
+      [transportKey]: {
+        wrtc // You can use `wrtc` when running in Node.js
+      }
+    }
+  }
+})
+```
+
 ## Configuration examples
 
 As libp2p is designed to be a modular networking library, its usage will vary based on individual project needs. We've included links to some existing project configurations for your reference, in case you wish to replicate their configuration: