Create configuration schemas for aggregator module

This is a sub-issue of #1910624: [META] Introduce and complete configuration schemas in all of core.

Problem/motivation

#1866610: Introduce Kwalify-inspired schema format for configuration introduced the idea of config schema. The changelog leads to (hopefully extensive) documentation on the format at http://drupal.org/node/1905070. While there are little cleanups planned for the format overall, the current format is a result of months of back and forths, so it should be perfectly fine to apply it more widely to core.

Proposed solution

Create a configuration schema for aggregator module.

Schema in place

Schema not yet in place
aggregator.settings.yml