Problem requiring files that already have inline source maps

I'm attempting to use `devtool` for a project that registers custom require handlers for Babel and CoffeeScript. These handlers inject inline source maps automatically, but these maps are being discarded by the `Module.wrap` override built into `devtool`.

I'll open a PR that skips source mapping when no `--break` or `--no-browser-globals` flags are specified, but correctly extending the source maps will take more time than I have right now.