Implemented the JSON writer for tag data and browser side javascript code.

Works with both http://example.com/tag/python+git and http://example.com/tag/python/git format. I will update the docs mentioning all this asap.

The only problem I faced was getting the `tags.json` file path within the `404.html` jinja template. Right now I have hardcoded the `tags.json` path in `404.html` to be on the website root. But if the user defines a prefix in the settings, then fetching `tags.json` would fail. Any idea on how to solve this?