Add host constraints to mix phx.routes output

[malteriechmann]

We use host constraints in router.ex, e. g.

scope "/", MyAppWeb, host: "api.example.com" do
  pipe_through [:api]
  get "/users", UserController, :index
end

scope "/", MyAppWeb, host: "admin.example.com" do
  pipe_through [:browser]
  get "/users", AdminUserController, :index
end

When running mix phx.routes we get:

GET  /users  MyAppWeb.UserController :index
GET  /users  MyAppWeb.AdminUserController :index

It would be better to get something like that:

[api.example.com]    GET  /users  MyAppWeb.UserController :index
[admin.example.com]  GET  /users  MyAppWeb.AdminUserController :index

If this is interesting for Phoenix framework, I would like to create a pull request for that.

[SteffenDE]

I don't see a reason why we shouldn't add it, so feel free to explore it and create a PR! :)