add support for progress option to provide a method on your model that will passed be notified of processing progress

StreamIO-ffmpeg supports a progress hook. All we needed to do was pass a block through to the transcode method.
It just goes in the options hash that you pass to encode_video, the value is a symbol that's the name of the the method on your model.

```ruby
    process encode_video: [:mp4, callbacks: { after_transcode: :set_success } , 
                           logger: :logger, progress: :progress]
```
I think it's a really useful feature if you're doing asynchronous processing using something like https://github.com/lardawge/carrierwave_backgrounder

If anybody has any ideas on cleaning up the implementation, please let me know.