Add issue (filter) to feed #6
Description
It should be possible to select an issue or set an issue filter for each feed. Currently all content goes into the latest issue, which might conflict with wanted behaviour.
Manually selecting an issue will suffice for basic use of Newscoop, but for more advanced use this is not enough. Therefore it should be possible to set an issue name filter. So when following issue naming guidelines automatically the correct latest issue is selected.