Support adapters for custom DB types

[Morozzzko]

ActiveRecord::SchemaDumper used in ActiveRecord::Annotate::Dumper doesn't work with custom PG enums.

It would be great to have some way to configure the class to allow easier integration with gems like schema_plus

https://github.com/SchemaPlus/schema_plus_enums/blob/master/lib/schema_plus/enums/middleware.rb

[Morozzzko]

Also, related issue: #7