Implement rudimentary package management commands

[joeduffy]

As soon as #5 is completed (the package manager protocol design), we should implement the basic package management commands (namely mu get and, ideally, mu list).

[joeduffy]

This is good for S9. Punting the rest to S10, as what we here do depends on #77.

[joeduffy]

I think for our March roadshow, we want to demo coco get installing from GitHub. So, pulling back just that subset into S9.

[lukehoban]

We've ended up building a makeshift package management solution using bash scripts for publishing/installing packs as part of our multi-repo build system. These have added a lot of boilerplate to each of our pack repos. We may want to consider turning this into a proper package manager exposed through the Lumi CLI sooner rather than later to simplify our own management of existing and new Lumi packs.