Proposal: add rigger commands for listing or viewing config / context

[krancour]

I'm proposing this as an alternative to user spelunking in ~/.rigger to find various configs / contexts for different clusters.

Related #52

Maybe if configs / contexts were put in ~/.rigger/contexts/my-cluster/vars (instead of ~/.rigger/my-cluster-vars we could provide something like rigger list-contexts (or list-clusters?) that just lists out the dirs in ~/.rigger/contexts.

We could even add something like rigger view-context my-context that just cats ~/.rigger/contexts/my-context/vars