cmd/kgctl: add output options for showconf

This commit adds several output options to the `showconf` command of the
`kgctl` binary:
* `--as-peer`: this can be used to generate a peer configuration, which
can be used to configure the selected resource as a peer of another
WireGuard interface
* `--output`: this can be used to select the desired output format of
the peer resource, available options are: WireGuard, YAML, and JSON.

cmd/kgctl/graph.go

@@ -25,7 +25,6 @@ func graph() *cobra.Command {
 	return &cobra.Command{
 		Use:   "graph",
 		Short: "Generates a graph of the Kilo network",
-		Long:  "",
 		RunE:  runGraph,
 	}
 }