Support for document separators

[elblivion]

Some of our specs use a valid multi-document YAML format:

kind: ClusterRole
apiVersion: rbac.authorization.k8s.io/v1alpha1
metadata:
  name: admin-access
rules:
  - apiGroups: ["*"]
    resources: ["*"]
    verbs: ["*"]
  - nonResourceURLs: ["*"]
    verbs: ["*"]
---
kind: ClusterRoleBinding
apiVersion: rbac.authorization.k8s.io/v1alpha1
metadata:
  name: kubelet-role-binding
subjects:
- kind: User
  name: kubelet
roleRef:
  apiGroup: rbac.authorization.k8s.io
  kind: ClusterRole
  name: admin-access

It seems like the YAML library used in kubeval doesn't support this yet: https://github.com/go-yaml/yaml#compatibility, so at the very least kubeval should warn the user that this is not supported (in testing, only the first YAML document in the file is parsed and the rest silently discarded).

[garethr]

I have some initial work on this in https://github.com/garethr/kubeval/pull/9 which seems to basically work. I want to clean it up, but if you have time and inclination I'd love any feedback.

For instance, using the fixture in this PR:

go run main.go fixtures/multi.yaml
The document fixtures/multi.yaml is not a valid Service
--> spec.ports.0.port: Invalid type. Expected: integer, given: string
The document fixtures/multi.yaml is a valid ReplicationController
The document fixtures/multi.yaml is a valid Service
The document fixtures/multi.yaml is a valid ReplicationController
The document fixtures/multi.yaml is a valid Service
The document fixtures/multi.yaml is a valid ReplicationController
exit status 1

[garethr]

Merged #9. Releasing an update now.