fill out the cli with useful commands

- [ ] help
- [x] add-maintainer/remove-maintainer
- [ ] namespace-create (need to talk about namespace-yank)
- [ ] add-to-namespace / remove-from-namespace
- [ ] abandon/yank package
- [ ] deprecate/yank package-version

Depends on:
https://github.com/entropic-dev/entropic/issues/52
https://github.com/entropic-dev/entropic/issues/53

Issue should probably get broken up but I wanted to record the work somewhere.

We should probably discuss our approach toward command design. As an opening salvo I will say that I prefer short words for commands and no elaborate options for the targets of those commands. Argument order should preferably _not_ be meaningful, or if meaningful unambiguous given existing context.

Example: `ds add-maintainer chrisdickinson namepace/pkg` is unambiguous in this example, but hard to predict without an example. Is it `ds add-maintainer target-package target-human` or does the human go first? Maybe it's great, though, because that last argument can be implicit if you're in a directory with a package.json.

`ds add-maintainer user=chrisdickinson pkg=namespace/pkg` is unambiguous but kinda weird.

`ds maintainers add chrisdickinson to namespace/pkg` is unambiguous if you speak English, might be hard to parse or translate.

`yank` is internal jargon that means two things: 'abandon' and 'deprecate'. Do we want to just let the jargon leak out into the user experience?