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'
  });
I'm not talking about the base name, I'm talking about a manifest file that's not in the root of the directory. It's not a huge problem for the default name, since that would not lead to much confusion, but I have a project with multiple files called `manifest.json`, in subdirectories, each manifesting something different. If my rev manifest was also called `manifest.json`, checking basenames would mean that any of those files could be confused for my manifest.