gulp-sourcemap support

[jGleitz]

Escodegen can generate sourcemaps. This is extremely helpful, because it allows to debug instrumented code in the browser.

This PR uses escodegen’s sourcemap feature if the user uses gulp-sourcemap and emits the map accodingly.

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This change is

[SBoudrias]

Let's just extend the options in the if statement. Then there's no need to do extra logic with the instrumenter as it'll just take in the options.

[jGleitz]

Before my changes, the instrumenter was created once for all files in the stream. My reasoning was that creating it might potentially be expensive. Thus, I wanted to only create a new instrumenter for each file if it’s really necessary (because we need to change its options).

Would you prefer something like this?:

var fileOpts = opts;
if (file.sourceMap) {
  fileOpts = _.defaultsDeep(_.cloneDeep(opts), {
    codeGenerationOptions: {
      sourceMap: file.sourceMap.file,
      sourceMapWithCode: true,
      sourceContent: fileContents,
      sourceMapRoot: file.sourceMap.sourceRoot,
      file: file.path
    }
});
var instrumenter = new opts.instrumenter(fileOpts);

[SBoudrias]

Yup, that would be cleaner IMO.

[jGleitz]

Done.

[SBoudrias]

Awesome thanks!