Updates pluck:recent rake task

Previously, running `rake pluck:recent` would pull down no more than the 20 most recent favorites not included in the database. This is fine 99% of the time considering that the task is run hourly by Heroku and the likelihood of favoriting more than 20 tweets in an hour is vanishingly small.

On the other hand, I don't have any such cron job running locally, and at times, I'd just like to update my local database to be current. In light of this, I've updated `pluck:recent` to accept an optional integer argument in (0, 200] that sets the maximum number of tweets to pull.

I also check to see if there are any favorites at all in the database. If there aren't, you can now use, say, `rake pluck:recent[100]` to seed a local database that you'd like to test with your 100 most recent favorites, rather than running `rake pluck:all` which will be more time consuming if you have more than 200 favorites.