Proposal: Add support for named contexts

[krancour]

Currently, when new configurations is generated using rigger configure, it is squirelled away in ~/.rigger/<generated hex string>/vars

It would likely be easier and more convenient for users to context switch between multiples Deis clusters if they could choose their own names for the context / configuration.

Proposal:

Create new config:

$ rigger configure my-cluster

^ Creates new configuration in ~/.rigger/my-cluster/vars

Create new config for a second cluster:

$ rigger configure your-cluster

^ Creates new configuration in ~/.rigger/your-cluster/vars

Switch back to the first cluster's config:

$ rigger use my-cluster