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.