Add utility to parse config and print out a key

Usage:

```
$ python -m synapse.config read server_name -c homeserver.yaml
localhost
```

synapse/config/__main__.py

+# -*- coding: utf-8 -*-
+# Copyright 2015 OpenMarket Ltd
+#
+# 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.
+
+if __name__ == "__main__":
+    import sys
+    from homeserver import HomeServerConfig
+
+    action = sys.argv[1]
+
+    if action == "read":
+        key = sys.argv[2]
+        config = HomeServerConfig.load_config("", sys.argv[3:])
+
+        print getattr(config, key)
+        sys.exit(0)
+    else:
+        sys.stderr.write("Unknown command %r", action)
+        sys.exit(1)