Add subcommands for image-repository

This adds all the standard subcommands for the ImageRepository type.

Following `source`, I have put them under a namespace: `auto`,
referring to automation.

NB For `create` I use controllerutil.CreateOrUpdate, which looks to me
like a slightly more rounded version of the upsert* funcs.

Signed-off-by: Michael Bridgen <michael@weave.works>

docs/cmd/flux_export_auto.md

@@ -0,0 +1,30 @@
+## flux export auto
+
+Export automation objects
+
+### Synopsis
+
+The export auto sub-commands export automation object in YAML format.
+
+### Options
+
+```
+  -h, --help   help for auto
+```
+
+### Options inherited from parent commands
+
+```
+      --all                 select all resources
+      --context string      kubernetes context to use
+      --kubeconfig string   path to the kubeconfig file (default "~/.kube/config")
+  -n, --namespace string    the namespace scope for this operation (default "flux-system")
+      --timeout duration    timeout for this operation (default 5m0s)
+      --verbose             print generated objects
+```
+
+### SEE ALSO
+
+* [flux export](flux_export.md)	 - Export resources in YAML format
+* [flux export auto image-repository](flux_export_auto_image-repository.md)	 - Export ImageRepository resources in YAML format
+