This adds `releases[].labels` in which you can set arbitrary number of key-value pairs, so that commands like `helmfile sync --selector key=value` can be used to run the helmfile subcommand against a subnet of declared releases.
`labels` and `selector` are named as such on purpose of being consistent with terminology of Kubernetes and other tools in the K8S ecosystem, including kubectl, stern, helm, and so on.
Resolves#8
Alex Withrow
Justin Nauman
rob boll