This uses `toml_edit` to edit the config file in-place. For the moment,
it only supports top-level arguments, but can be extended to do other
things later.
Universes are really an optimization of filtering tasks, so let's define
them there, and derive them from the set of conditions. This means that
complex filters might get missed and end up doing a full task scan, but
that's probably OK.
Note that this does not fix the working-set API issues (#108 and #123).
* 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
