class Twigg::Settings
Simple OpenStruct subclass with some methods overridden in order to provide more detailed feedback in the event of a misconfiguration, or to provide reasonable default values.
For example, we override `repositories_directory` so that we can report to the user that:
- the setting is missing from the configuration file - the setting does not point at a directory
We override `app.bind` so that we can supply a reasonable default when the configuration file has no value set.
In order to keep this class a simple and readable manifest of the different options, the real logic is all implemented in the {Twigg::Settings::DSL} module, leaving only the DSL
declarations here.