Broccoli concatenator that generates & propagates sourcemaps

This filter is designed to be fast & good enough. It can generates source maps substantially faster than you'll get via mozilla/source-map, because it's special-cased for straight line-to-line contenation.

It discovers input sourcemaps in relative URLs, including data URIs.

var tree = concat(tree, {
  outputFile: '/output.js',
  inputFiles: ['loader.js', '**/*']
});