Add list admins commandline tool. (#170)

* adding a list_admins command

* updating README and V2 ideas to include list_admins

* fix documentation misses

* add list_admins to the toolchain

bin/blackbox_list_admins

@@ -0,0 +1,8 @@
+#!/usr/bin/env bash
+
+#
+# blackbox_list_admins -- List authorized admins
+#
+set -e
+source "${0%/*}/_blackbox_common.sh"
+cat "$BB_ADMINS_FILE"