Allow passing list of permissions modules

Hi,

Maybe I missed this but it is possible to compose the setup of Permit by passing a list of permissions modules instead of just one ? Ideally I would like to have a permissions module/file for each context.

```elixir
defmodule MyApp.Authorization do
  use Permit, permissions_modules: [MyApp.Accounts.Permissions, MyApp.Orders.Permissions, ... ]
end
```