Read JSON manifest files

This pr adds a `manifest` option that allows a user to specify JSON manifest files produced by `rev`. Usage example:

```javascript
gulp.src('index.html')
  .pipe(revReplace({manifest: gulp.src('rev-manifest.json')}))
  .pipe(gulp.dest('build'));
```

The rationale for the addition is that in larger projects it's common to process files in separate tasks, or even incrementally. In such cases there are no `.revOrig*` properties for gulp-rev-replace to read in the stream, but there are manifest files that contain that same information.

Includes a test and readme update. Let me know if there's anything stylistically you don't like.