Add a --context option

Signed-off-by: Sylvain Rabot <sylvain@abstraction.fr>

docs/cmd/flux_delete_source.md

@@ -15,6 +15,7 @@ The delete source sub-commands delete sources.
 ### Options inherited from parent commands
 
 ```
+      --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")
   -s, --silent              delete resource without asking for confirmation