Pass an argument to block for 'delete'

I couldn't get past an error saying "PStore::Error at /0" saying that it couldn't delete my idea w/ an id of 0 from my database. We weren't passing anything into that method as a param...the change to the code below has it working.

### Add `delete` to `Idea`

We can fix this by adding a `delete` method in `idea.rb`:

```ruby
class Idea
  def self.delete(position)
    database.transaction do **|db|**
      db['ideas'].delete_at(position)
    end
  end
end