Generate CRDs from store and Knative serving Service

Changelog:

• Generate CRD from store images #637
• Add knative serving v1alpha1 to generate CRD command #638

Example commands:

# Generate CRD entry for a store function
$ faas-cli generate --from-store figlet
---
apiVersion: openfaas.com/v1alpha2
kind: Function
metadata:
  name: figlet
  namespace: openfaas-fn
spec:
  name: figlet
  image: functions/figlet:0.13.0

# Generate a CRD entry to deploy to Knative serving
$ faas-cli generate \
    --api serving.knative.dev/v1alpha1 \
    --from-store figlet

---
apiVersion: serving.knative.dev/v1alpha1
kind: Service
metadata:
  name: figlet
  namespace: openfaas-fn
spec:
  runLatest:
    configuration:
      revisionTemplate:
        spec:
          container:
            image: functions/figlet:0.13.0