Add hierarchic properties to tags (Feature Request)

#### Feature:
Make Tags hierarchical in the same way that projects are.

In my use case I use tags for tracking and the teams I work with and the type of work I do. For teams this allows me to quickly review my upcoming and historic activities with another team. I track the type of work so I can see how much time I am spending `developing` vs. time spend in `meeting`s.

#### Hierarchy would be a welcome addition for the following reasons.
* Since some of the team names are also the same as other tasks I need to nest them under a tag "heading" I need to be able to nest team names under a sub-heading in my tag. For example while I have a `development team` I also have tasks which are tagged `development` because they are development tasks. So, I have two tags `team.dev` and `dev`.
* I work with sub-teams (i.e. `team.it`, has `team.it.systems`, `team.it.networking`, `team.it.helpdesk`, etc.) but want to be able to query for all work with `team.it` including those sub-teams. Currently this requires searching for all possible sub-teams. I would like to be able to just search for `team.it` and get all the underlying tasks

#### For example, If I had the following two tasks.

```
ID Age Deps Project Tag Due Description Urg
1 2d website.frontend team.it team.design Web Host Move Meeting 10.8
2 1d training team.it.helpdesk meeting Security Training 14.2
```

With only the few IT subteams mentioned above it becomes quite complex to filter for the tasks that I want. If I want to create a report of all my upcoming tasks with the IT team I currently have to search in the following way.

`task +team.it or +team.it.helpdesk or +team.it.systems or +team.it.networking`

Adding hierarchy would change this search into the following search.

`task +team.it`

Supporting this use-case will open up opportunities for taskwarrior to be easily used to track and report on more than simply what tasks were completed. It will allow easy categorization and reporting on how those tasks were completed, with whom they were completed, or any range of other contextual tracking that a user may desire.