Wrap cache manifest file contents with vinyl File

Hi, thanks for this nice plugin. Now gulp plugins are [expected to output vinyl File objects](https://github.com/gulpjs/gulp/tree/master/docs/writing-a-plugin). So I modified this plugin to do so.

Also, the following relevant fixes are included.

- Added some basic tests.
- Fixed broken `exclude` support.
- Made the paths of fake files absolute. `gulp.src()` outputs absolute paths.