Adding listen-port option to `mix mobilizon.instance gen`

lib/mix/tasks/mobilizon/instance.ex

@@ -24,6 +24,7 @@ defmodule Mix.Tasks.Mobilizon.Instance do
   - `--dbname DBNAME` - the name of the database to use
   - `--dbuser DBUSER` - the user (aka role) to use for the database connection
   - `--dbpass DBPASS` - the password to use for the database connection
+  - `--listen-port PORT` - the port the app should listen to, defaults to 4000
   """
 
   use Mix.Task
@@ -48,7 +49,8 @@ defmodule Mix.Tasks.Mobilizon.Instance do
           dbname: :string,
           dbuser: :string,
           dbpass: :string,
-          dbport: :integer
+          dbport: :integer,
+          listen_port: :string
         ],
         aliases: [
           o: :output,