add optional PVC

[jessebot]

fixes #28. Adds the following parameters:

persistence:
  # -- enable persistence for the pixelfed pod
  enabled: false
  # -- storage class name
  storageClassName: ""
  # -- size of the persistent volume claim to create. Tgnored if persistence.existingClaim is set
  storage: 2Gi
  # -- accessMode
  accessModes:
    - ReadWriteOnce
  # -- using an existing PVC instead of creating one with this chart
  existingClaim: ""