Upload to source.coop command?

[cholmes]

Ok, this one I'm pretty sure belongs in Ceruleo, but we don't have a repo for that yet - probably should start one to gather ideas. But following on #80 it'd be nice to have a command that fully creates a new source.coop data product, and updates STAC, README and 'description' (which I'm hoping we can create with a source.coop API, so they're all consistent (I wish source would do more of this syncing between the three on its own, but Jed has seemed a bit resistant to that in the past).