Add possibility to use another outputter (like Rails' logger)

At work, our logs are in the Rails logger files and I only look at these (we have almost nothing 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 on my `log/development.log` is

![screen shot 2014-04-02 at 23 19 53](https://cloud.githubusercontent.com/assets/236461/2597670/957af7f0-bab7-11e3-98e3-a3b4016a6b55.png)