Don't use a slice of pointers to opml items, when we can simply use a slice of
items instead. This should reduce the amount of memory allocations and the
number of indirections the GC has to process, speedup up the import process.
Note that this doesn't introduce any additional copies, as the only time a
slice of subscription is created, the items are created and inserted inline.
This adds a new "description" field to the feed settings. This allows to
save custom description regarding a feed. It is also exported and
imported as "description" in OPML.
