Add linked tag support

The following PR shows how to add linked tag support to Ghostium. Feel free to close this and implement it in a different way if preferred. I just wanted to demonstrate what the required changes are, and PR was easiest :smile:

closes #78

- Changed the html wrapping each tag to be an `<a>` instead of a `<span>` - not sure if the span was needed for some other purpose?
- Added a href using the `{{url}}` helper
- Prefixed if with a `~` to collapse whitespace