Locate PropertySourcesLoaders using SpringFactoriesLoader and refactor
the interface to expose file extensions and support 'profiles' within
documents.

Rework ConfigFileApplicationListener for consistent profile loading.
Profiles are now loaded in a consistent order for both profile specific
files, and contained profile documents (i.e. YAML sub-sections).

Also update ConfigFileApplicationListener so that it no longer directly
processes @ProperySource annotations. Instead the standard Spring
ConfigurationClassPostProcessor will insert @PropertySource items with
ConfigFileApplicationListener later re-ordering them.

The SpringApplication can no longer be configured using @ProperySource
annotations, however, application.properties may still be used.

Fixes gh-322