Combine Previous Manifest with Current

- When `rev-manifest.json` is included in `gulp.src`, the old manifest
  will be used to preserve data. This allows separate gulp tasks to
  use the same manifest.
- Optionally supply a file name to `rev.manifest`. Defaults to
  'rev-manifest.json'. Example: `rev.manifest({ name: 'manifest.json'
  });
If you have a file in your stream called `bar/rev-manifest.json` and your manifest's name is `foo/rev-manifest.json`, checking the basename will result in a false positive.