All #output method to accept a block which is passed each line from the CLI output. 

* I apologize that there are no tests associated with this change.
* This new feature allows for better control over the output. (see example below)

```ruby
bar = ProgressBar.new("Handbrake", 100)
handbrake.input(tmp_mkv_file.to_s).preset('High Profile').output(tmp_m4v_file.to_s) do |line|
  match = line.match(/Encoding:.*\s([0-9\.]+)\s%/)
  bar.set( percentage = match[1].to_f ) if match
end
bar.finish
```

Thanks for making this great wrapper gem!