Zopfli worker

This is initial idea of a Zopfli worker, but after working with it I am not sure it fits into the idea of image_optim.

Zopfli creates a .gz version of the png file, which seems to be incompatible with png openers (Which makes sense).

I took some ideas from https://github.com/mathiasbynens/grunt-zopfli/blob/master/tasks/zopfli.js

```ruby
brew install zopfli
zopli -i15 image.png # this creates image.png.gz

zopli -c -i15 image.png > image.png # this creates image.png but it is gzipped, so nothing will open it
```

Do you have any idea how to approach this?

This references issue https://github.com/toy/image_optim/issues/17