Allow base class for migrations to be configurable

Currently, all data migrations inherit from `DataCustoms::Migration`. It can be useful to configure that base class to something like `ApplicationDataMigration` to add shared behavior. A rough sketch of what it could look like:

```rb
class ApplicationDataMigration
  include Rspec::Matchers
end

DataCustoms.configure do |config|
  config.base_class = "ApplicationDataMigration"
end
```
