adding ability to configure multiple workers in the routes file

Example:

```ruby
  topic "cheese" => CheeseConsumer, :workers => 3 do
    map "cheese.new" => "eat"
    map "cheese.updated" => "try_again"
  end

  topic "beer" => BeerConsumer do
    map "beer.new" => "drink"
    map "beer.deleted" => "ask_for_more"
  end
```