Add a --context option

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

docs/cmd/flux_create_source.md

@@ -15,6 +15,7 @@ The create source sub-commands generate sources.
 ### Options inherited from parent commands
 
 ```
+      --context string      kubernetes context to use
       --export              export in YAML format to stdout
       --interval duration   source sync interval (default 1m0s)
       --kubeconfig string   path to the kubeconfig file (default "~/.kube/config")