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 path to `rev.manifest`. Defaults to
  'rev-manifest.json'. Example: `rev.manifest({ path: 'manifest.json'
  });
Merge pull request #51 from primedia/appendable_manfist

Combine Previous Manifest with Current