Add a --context option

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

docs/cmd/flux_export_source_git.md

@@ -31,6 +31,7 @@ flux export source git [name] [flags]
 
 ```
       --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)