Added a configuration delegate to ignore some endpoints in the index

Add a new method on SwaggerSpecConfig which allows the end user to specify an arbitrary delegate which is used to determine whether or not an endpoint should be included in the index.  Includes a new test for this feature.
The method signature is a little confusing here - I would suggest "IgnoreOperationsWhere", where the ignoreResolver returns true if the operation should be ignored. It seems to work the other way round now, i.e. it returns true to keep the operation.