* Use a parser (rather than clap) to process the command line
* Outline some generic support for filtering, reporting, modifying, etc.
* Break argument parsing strictly from invocation, to allow independent testing
The public API of the taskchampion crate now contains the expected parts
and no more, and has some better documentation.
This moves the crate's external `tests/` into internal tests, as the
TaskDB is no longer exposed as part of the crate API.
