Pass custom options to slimrb command

Slim has big list of options that you can pass to ```slimrb``` command with ```-o``` flag:
https://github.com/slim-template/slim/blob/master/README.md#available-options
But there is no way to do it in current implementation of ```gulp-slim```.
This pull-request fixes it.

Now you can pass any options that you may need:
```javascript
slim({
  pretty: true,
  options: 'attr_quote="\'"'
})

slim({
  pretty: true,
  options: ['attr_quote="\'"', 'js_wrapper=:cdata']
})
```