Add a "shell" pseudo-command to nomulus tool

Add the "shell" command which lets you run multiple other command in a single
session, sparing you the initialization costs for all but the first of them.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=188712815

java/google/registry/tools/CommandRunner.java

@@ -0,0 +1,24 @@
+// Copyright 2018 The Nomulus Authors. All Rights Reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package google.registry.tools;
+
+/**
+ * Interface for things that run commands.
+ *
+ * <p>This exists only to allow us to test ShellCommand.
+ */
+interface CommandRunner {
+  public void run(String[] args) throws Exception;
+}