Create a hook_feeds_processor_targets().

Currently we have, hook_feeds_processor_targets_alter() but no hook_feeds_processor_targets(). This makes some sense if you figure that targets are initially defined by the Processor class, and additions are made. But, as seen in #661606: Support unique targets in mappers we need an actual alter function. One that can assume that all of the targets have been added, and can actually alter existing ones.

This can be added in a backwards compatible way, since hook_feeds_processor_targets_alter() will still behave as normal. Only core Feeds functions need to be changed to use the new api. AND the docs need to be updated. Other modules could update whenever they felt like it.