Add initContainers

Could we please add initContainers?

Inspired by how helm/stable/nginx-ingress does it:

• https://github.com/helm/charts/blob/master/stable/nginx-ingress/values.yaml#L319-L323
• https://github.com/helm/charts/blob/master/stable/nginx-ingress/templates/controller-deployment.yaml#L205-L208

Usage example:

$ echo "extraInitContainers:\n- name: init\n image: busyb ox" | helm template -f - . | grep initContainers -C3

          timeoutSeconds: 3
        resources:
            requests: null
      initContainers:
        - image: busybox
          name: init
---