Create ignores collection from factory

Create the collection of ignores to filter by the factory method. If it
is null, use an empty list.
Settings file does not contain ignore list

When the settings file doesn't contain an ignore list, we should default
to this list.
Handle empty array of ignores in settings

When the ignorables array in the settings is empty, we should handle it
by not specifying any ignores at all.
Handle the list of ignores in the settings file

When ignorables are specified in the config file, we should add those.

Also refactored out the code which creates the ignorables into a
separate method.