New plugin to get a 'Random' post

I created a new plugin to get a 'Random' post when hitting /random/ url.
Nikola [generates](https://github.com/getnikola/nikola/blob/master/nikola/plugins/task/tags.py#L111) a json file for tag clouds. See `output/assets/js/tag_cloud_data.json`.  You could use that json file instead of generating your own.