Prevent from grouping by extension

This example taken from the README.md didn't work:

```js
gulp.src('./bower.json')
  .pipe(inject(gulp.src(['./dist/app.min.js', './dist/app.min.css'], {read: false}), {
    starttag: '"main": [',
    endtag: ']',
    transform: function (filepath, file, i, length) {
      return '  "' + filepath + '"' + (i + 1 < length ? ',' : '');
    }
  }))
  .pipe(gulp.dest('./'));
```

It would only inject `app.min.css` since the injection of the CSS overwrites the injection of the JS.

I've added `ignoreExtensions` option to make it possible to inject all files regardless of their extension (could be useful in many situations I think).