Add svgo worker for optimizing SVG

Adds svgo (available from npm) as a worker, and also adds the SVG extension as a valid image type.

@toy, I monkey patched my version locally to ensure this works, but it requires the latest commit on image_size. Since add_dependency doesn't accept a git source, can you release a new version so I can bump the gem version in the gemspec?

Closes https://github.com/toy/image_optim/issues/27

Thanks!
~ Jonathan