Parameterized type for `Algae.Maybe`

Hi!

I think it would be useful to have a parameterized type for optional entities, e.g.

```elixir
@spec f(Algae.Maybe.t(String.t())) :: any
def f(opt_name) do
  # ...
end
```

We could simply add a `@type t(x) :: ...` to `Algae.Maybe`, but ideally `defdata` would take care of it. Is it possible?