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