Draft: improve JavaScript, add React Native

FC (Fay) Stegermanrequested to merge
obfusk/diffoscope:js into master

Addresses #109 to some extent.

I've proposed a patch for file(1) JavaScript detection, including React Native.

Regular JavaScript should continue to work fine w/o the patch (since I've included \btext\b in FILE_TYPE_RE for now), but ReactNativeJavaScriptFile detection will not work without it.

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