Add an option to format tag list as a tree

Problem/Motivation

Currently no hints are provided as to what the relationships of tags are to each other. Showing the tags (terms) in tree format (with or without counts) would inform the user about the semantics behind tags. A tree display can also serve as a reference for content editors which tags are available for use. Placed on a sidebar, it makes a great table-of-contents-like navigation widget.

I've hacked a custom block for this, but actually this would much better fit into this module, which already includes everything needed except the markup formatting.

Proposed resolution

Hierarchically sorting tags provided by the current query and rendering a nested HTML list is all that is needed. I'll behappy to provide the sorting functions, if this enhancement is deemed useful.