Add code-server-traitlet example. (#148)

Add code-server-traitlet example.

Author: betatim

contrib/code-server-traitlet/README.md

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+## code-server-traitlet
+
+Create a code-server launcher [via traitlet](https://jupyter-server-proxy.readthedocs.io/en/latest/server-process.html#specifying-config-via-traitlets).
+
+Via https://github.com/betatim/vscode-binder.

contrib/code-server-traitlet/jupyter_notebook_config.py

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+c.ServerProxy.servers = {
+  'code-server': {
+    'command': [
+      'code-server',
+        '--no-auth',
+        '--disable-telemetry',
+        '--allow-http',
+        '--port={port}'
+    ],
+    'timeout': 20,
+    'launcher_entry': {
+      'title': 'VS Code'
+    }
+  }
+}