Add a means of retrieving the default audits

This adds a new defaults() class method to the cognosco.audit.Auditor
class.  The new method returns a sorted list of audit names, and may
be used to display, in help text, a list of the default audits that
Cognosco will run.
Retrieve default audits from context

This allows the default audits to be derived from the context.  If they
aren't specified in the context, or if they're specified as an empty
list, then the previous defaulting behavior will be used.