Allow other cmd line args

@jnunemaker are you thinking something more like this (over https://github.com/jonmagic/grim/pull/18)?

```ruby
pdf[3].save('/path/to/image.png', :extra => ["-flatten", "-background white"])
```

I'm passing it into the options hash in the `:extra` key because it felt weird any other way I tried.

/cc @augustt198 @yanlinpu