Add jsx_transformer_js option

[a2ikm]

It forces the path to JSXTransformer.js.

Currently assets.resolve("JSXTransformer.js") seems to search in tmp/react-rails, vendor/assets/react and vendor/assets/react/development at first.

So it's hard to place custom version in other directories like vendor/assets/bower_components managed with bower.

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[rmosolgo]

@a2ikm thanks for taking a first pass at this, sorry some other changes have caused merge conflicts!

I took a stab at implementing this in #273, so you could pass it as a config option:

 config.react.jsx_transform_options = {
   asset_path: "bower_components/path/to/JSXTransformer.js" 
 }

What do you think of that?

[rmosolgo]

I've merged #273, you can pass a custom asset_path: to jsx_transform_options!

[a2ikm]

@rmosolgo Sorry for my late reply. Your patch looks very nice, thank you!