Builder configuration

[peteryates]

The form builder currently makes several choices on things like default label sizes, legend sizes, tags and the localisation schema (#68). There should be an option to override them so that the builder can be made to work better with a variety of projects.

Ideally the config should look something like this:

GOVUKDesignSystemFormBuilder.configure do |config|
  config.default_label_size          = 'm'
  config.default_legend_size         = 'l'
  config.default_legend_tag          = 'h3'
  config.default_localisation_schema = %i(activerecord attributes)
  config.default_submit_button_text  = 'Make it so'
end

• Build the ability to load and use configuration, ensure it works when run from a Rails initialiser
• Integrate label and legend formatting options with the configuration
• Document and provide examples