Experimental support for embedded source maps

It's barely working but I wanted to get input on it as soon as possible. Here's an example gulp task:

```javascript
gulp.task('scripts', function() {
  var bundler = watchify(path.join(__dirname, 'app/scripts/main.js'));

  bundler.on('update', rebundle);

  function rebundle() {
    return bundler.bundle({ debug: true })
      .pipe(source('all.js'))
      .pipe(streamify(sourcemaps.init()))
      .pipe(streamify(uglify()))
      .pipe(streamify(sourcemaps.write('.')))
      .pipe(gulp.dest(path.join(__dirname, 'out/assets')));
  }

  return rebundle();
});
```