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!

Let's make this slightly more flexible in case whitespace changes. It's pretty unlikely we'll eve add it before the colon but after is pretty reasonable (in fact, I may go get somebody to fix that because it's bothering me now :D).