Aggregator should support filter.module

The aggregator module in 4.5 and 4.5.1 hardcodes the default "Filtered HTML" filter, as opposed to hooking into the filter.module system itself. This can cause problems for a number of feeds (see http://drupal.org/node/13283 for an example), since [IMG] is not one of the allowed tags. It also greatly causes issues for users of Flickr (see http://flickr.com/forums/help/2943/ for a Drupal specific issue). If aggregator.module could hook into the default filter.module, users would be able to create a "Aggregator" filter which includes tags they'd like to see (like [IMG]). The first stage of the feature should be "support a single filter for all aggregated items" and the second stage should be "support a specific filter for a specific feed, with a default for all unspecified". The default, out of the box, behavior, should be to use the "Filtered HTML" filter.