Make it possible to drop in non-release React

[zpao]

As master diverges from releases, it would be great to make it possible to drop in a specific version of React that overrides the files provided by react-source.

• use react.js, react.min.js
• use JSXTransformer.js to do transformation (important as it may diverge)

[jtmalinowski]

The easiest way I suppose is to register vendor after default sprockets paths are added, then due to the way priorities are done, assets.resolve('react.js') will return user's react if it exists within the project, or ours' otherwise. Same with jsxtransformer.

Some potential problems as I started implementing:

1. We always use react.js and depending on variant, either react.js, or react.min.js is available under this path. There is a certain discrepancy here, because end-user cannot do the same, i.e. he will have to use the same version of react for dev/production
2. Do we want to test this? If so, probably the best way would be to duplicate dummy app, because otherwise we would have to mess with assets files inside tests.

[zpao]

Sorry I didn't catch you before you started, but I appreciate you taking this on.

We always use react.js and depending on variant, either react.js, or react.min.js is available under this path. There is a certain discrepancy here, because end-user cannot do the same, i.e. he will have to use the same version of react for dev/production

I commented on the PR but I think we can solve this.

Do we want to test this? If so, probably the best way would be to duplicate dummy app, because otherwise we would have to mess with assets files inside tests.

Yes. We can probably just put nonsense JS in those test files so we can test them easily - doesn't necessarily need to actually be React. Another dummy app seems fine (a lot of extra files for such a simple test, but if that's what we need to do, let's do it).

[jtmalinowski]

This is implemented in #12