Add domain option for tunnel creation (#312)

* Add domain option for tunnel creation

* Update documentation for the static free domain

README.md

@@ -291,6 +291,19 @@ The `bind_tls` option is now `scheme`. When `bind_tls` was true (the default), n
 
 The `auth` option, also available as `httpauth`, is now just `basic_auth`. Note also that the password for `basic_auth` must be between 8 and 128 characters long.
 
+### Using the ngrok static free-domain
+
+In April 2023, ngrok [introduced the ngrok static free-domain](https://ngrok.com/blog-post/new-ngrok-domains), which brings you a static subdomain even for free accounts. To make use of it, just use the `domain` option when creating a tunnel. Example:
+
+```ts
+ngrok.connect({
+  domain: "xxx.ngrok-free.app"
+  authtoken: "YOUR-AUTH-TOKEN"
+})
+```
+
+__Note__: Please make sure to __either__ use the `subdomain` or the `domain` option. Using both would lead to using the subdomain only.  
+
 ## Upgrading to version 4
 
 The main impetus to update the package was to remove the dependency on the deprecated `request` module. `request` was replaced with `got`. Calls to the main `ngrok` functions, `connect`, `authtoken`, `disconnect`, `kill`, `getVersion` and `getUrl` respond the same as in version 3.

src/utils.js

@@ -64,6 +64,7 @@ const tunnelProperties = [
   "key",
   "terminate_at",
   "labels",
+  "domain",
 ];
 
 const globalProperties = [