Implement source maps

[jRiest]

Here's a basic implementation of source maps (fixes #3).

jsx-loader isn't emitting source maps yet but I have pull request open (petehunt/jsx-loader#19) which would add that. With this pull request it will pick up the source map whenever jsx-loader sends it, otherwise it'll just proceed without a source map like it does currently.

In order to keep the mapping really simple, I switched all __hotUpdateAPI references to __HUA in order to match the character length of React. This simplifies that transform so we don't have to update the source mappings that come from jsx-loader wherever we make a replacement, but we could do that instead if necessary.

[gaearon]

That's amazing! Nice idea with __HUA 👍
I'll test it on my project and hopefully merge today!

[gaearon]

Sorry for the long delay. Ideally I'd like to merge this after petehunt/jsx-loader#19 gets through.

Would you mind adding this.sourceMap check?

[jRiest]

No problem. If this.souceMap === false source maps will no longer be emitted.

[gaearon]

Since we're waiting for petehunt/jsx-loader#19 and I had to move all hot loader code to top, I decided to collapse all hot loader code into one line so it doesn't obscure each file. This isn't as nice as source maps, but on the upside it works with devtool: eval (which is faster).

This will require a few adjustments in your PR but before doing them, let's wait for JSX source map support to land.

[gaearon]

Thank you again! Source maps are in 0.5.0.
I needed to tweak this PR so I did it via a separate commit, but it's really thanks to you that we have this.