Initialize @feeds when account is empty

If the Superfeedr account isn't subscribed to any feeds when Hubot boots, we
get back no data and the loop in `parse_feed_callback` never runs, which left
`@feeds` uninitialized. Now we initialize it to `[]` first thing, so `feed
list` and `feed add` will work.

Fixes LeonB/hubot-superfeedr#1