Cleanup the fallback item config loading code.
Loader's config property should reflect the ini_config.

(Also, bring the fallback ini loader into compliance with the interface.)
Added basic filter support (filter-with in app config)

pipeline and filter-app are not yet supported.
nested filter-withs are kind of annoying; we have to return a wrapper.
also, we don't yet adapt paste.filter_app_factory, because none of the tests use it.