Roadmap

Replace Group objects (Today, SortedTimeGroup, SortedTaskGroup) with a single generic group object
If there are not enough tasks on Today, add some from Anytime
Auto-detect tags and tag relationships on sorting, instead of relying on TAG_CATEGORIES configuration Ideally, pass in the names of the parent tags in the order to sort by. e.g. bin/today 'when:,what:' Currently this script does the following:

Reading from Things.app

Groups task types together, i.e. errands, focussed work etc. Because obviously I want to do all my phone-calls, emails etc. at the same time, and order by the usual order I do things in (chores, focussed work, errands ...) 
Group any tasks that have been given a time of day tag. i.e. If a task has to be done in the morning I tag it "when:morning", it groups all of these together
Make sure all tasks are grouped by task type within their time of day grouping, i.e. make sure all my morning-errands are grouped together
Order the tasks within task type groups by importance, i.e. most important errand goes first - Order the groups of tasks ordered by type by group importance. i.e. If I have a bunch of errands to do, and a bunch of admin, given I have a very important errand in the errand group, then the whole errand group comes before admin.