Restructuring: Isn.Extension is now called Isn, which means that the postgrex
configuration line becomes a bit shorter now:
```elixir
{:ok, pid} = Postgrex.Connection.start_link(
database: "isn_test",
extensions: [{Isn, {}}])
```
instead of the previous version, with `extensions: [{Isn.Extension, {}}]`.
I also implement the entire library in the `lib/isn.ex` file, since it felt
pretty silly to have eight different one-line files under `lib/isn/` looking
like this: `defmodule Isn.ISBN, do: use(Isn.Base, :isbn)`
We now have the following types:
- ISBN
- ISBN13
- ISMN
- ISMN13
- ISSN
- ISSN13
- EAN13
- UPC
Martin Frost
Igor Barchenkov
Martin Frost