Add possibility to use another outputter

At work, our logs are in the Rails logger files, not on STDOUT. This PR allows to use Priscilla with the Rails logger (or any other outputter).

### Default behavior (unchanged)
![screen shot 2014-04-02 at 23 16 57](https://cloud.githubusercontent.com/assets/236461/2597622/f079dbea-bab6-11e3-971c-ab0d7edef1a6.png)

### Configured behavior
With this configuration:
```ruby
Priscilla.configure do |c|
  c.outputter = ->(s) { Rails.logger.debug s }
  c.blank_line_before = true
  c.blank_line_after = false
end
```

The result is

![screen shot 2014-04-02 at 23 19 53](https://cloud.githubusercontent.com/assets/236461/2597670/957af7f0-bab7-11e3-98e3-a3b4016a6b55.png)