feat(React::JSX): add JSX.transform_options

I want the feature from #107  :)

Now you can say in `application.rb`:

```ruby
config.react.jsx_transform_options = {harmony: true}
```

And you'll get es6 features in your `.jsx` assets. What do you think about this implementation?

- Take a config
- Assign it to a module variable 
- Use that when rendering with `React::JSX::Template`

I had to change the requires in the dummy `application.js` so that the ES6 file wouldn't wreck other tests.

I've used it with a test component in my app and it works!

Is there a particular advantage to make this `mattr_accessor` vs storing these options in a more local place?

Also AFAICT `mattr_*` come from Rails. I suppose that's ok in a rails-specific gem, though I have thought about pulling the rendering out. We could cross that bridge if we ever get to it.