This refactors GTIN handler class lookup so it can be extended without
monkeypatching. Developers can add/remove classes in
`BarcodeValidation::GTIN.gtin_classes` to control which class wraps a
GTIN. The use case I need this for is internall dummy GTINs that are
longer than any of the standard ones.

The Readme is updated to describe the feature.

This is part of marketplacer#29, which makes it easier to implement custom GTINs as
a user of this library. The first implementation in a0d0e9b worked, but
required the user to directly manipulate registered classes inside an
Array. As @beet pointed out, it's better to not expose implementation
details like that and look for a more elegant interface.

This commit changes the implementation by replacing `gtin_classes` with
`prioritized_gtin_classes` and adding a note that this is a private API.
Any subclass of `GTIN::Base` will be automatically appended to this list.

The user gets two tools directly inside `GTIN::Base`:

- `prioritize_before (other_class)`: moves this class to directly in
  front of the other class. This allows this class to be asked if it
  `handles?` a GTIN before the other class.
- `abstract_class`: lets a class remove itself from the list of classes
  that are asked if they handle a GTIN.

There are tests for both the implementation details and functionality.

The goal is to make `prioritized_gtin_classes` private data and only
expose methods that have interactions with it. Replacing #delete
behavior from GTIN::Base with calls to this new method is one step in
the right direction.

This is another small step towards making GTIN::Base not manipulate the
internals of `GTIN.prioritized_gtin_classes`, but letting GTIN handle
that by itself.

Manipulating `GTIN.prioritized_gtin_classes` now only happens through
helper methods on GTIN, theoretically allowing the list itself to become
private.

Having a few lines to outline the purpose of a class and its neighbours
tends to help understanding and exploration of a codebase.

The only thing that still depended on it being public were the tests for
functionality that manipulated the list. The tests have been overhauled
to rely more on asserting behavior than internal state.