Make Adapters registerable so they are not namespace-constrained

Changes:

- Introduce Adapter::get for use by Serializer.adapter
- Move Adapter-finding logic from Adapter::adapter_class into Adapter::get

Introduced interfaces:

- non-inherited methods
```ruby
ActiveModel::Serializer::Adapter.adapter_map     # a Hash<adapter_name, adapter_class>
ActiveModel::Serializer::Adapter.adapters        # an Array<adapter_name>
ActiveModel::Serializer::Adapter.register(name, klass) # adds an adapter to the adapter_map
ActiveModel::Serializer::Adapter.get(name_or_klass)    # raises Argument error when adapter not found
```

- Automatically register adapters when subclassing

```ruby
      def self.inherited(subclass)
        ActiveModel::Serializer::Adapter.register(subclass.to_s.demodulize, subclass)
      end
```

- Preserves subclass method `::adapter_class(adapter)`

```ruby
      def self.adapter_class(adapter)
        ActiveModel::Serializer::Adapter.get(adapter)
      end
```

- Serializer.adapter now uses `Adapter.get(config.adapter)` rather than have duplicate logic

.rubocop.yml

@@ -16,6 +16,9 @@ Lint/NestedMethodDefinition:
 Style/StringLiterals:
   EnforcedStyle: single_quotes
 
+Style/SpecialGlobalVars:
+  Enabled: false
+
 Metrics/AbcSize:
   Max: 35 # TODO: Lower to 15