Allow to order feeds/groups of feeds in ascending/descending order separately

Following some thoughts on issue #419, I think there is a need to separately configure feeds or groups thereof to be sorted in chronological or reverse-chronological.

For example,
- for a group of feeds from news websites (e.g., the Guardian or Slashdot), it is more relevant to get to the latest articles first, as the earliest ones are staler [on a side note, it might make sense to time-out older items, but it's probably a job for the oC app itslef],
- for a group of feeds from webcomics or other serials (e.g., podcasts), you want to read the earliest first, as the old ones won't make sense otherwise.

So, rather than an application-wide configuration of the order to display feeds in, this should rather be a per-feed or per-group setting.