REPL: Source map viewer

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EDIT: http://sokra.github.io/source-map-visualization/

Would be interesting to be able to know how different the input/output source is from Babel. Like what if you could see visually the difference of `async function a() {}` and the output? You could calculate the bytes change, and even suggest a change in config (loose mode, different transform, target different browser) and the % difference in size

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> My feature request would be a source map output view. It would be great to easily see the maps that babel creates for different configs

https://twitter.com/jasonlaster11/status/893914807090520064 via @jasonlaster

Similar to https://twitter.com/left_pad/status/885547523233984512 - we should integrate this too @boopathi