plugins: Add simple helloworld plugin to test against

It does double duty to show how a simple plugin might work.

contrib/helloworld-plugin/main.py

@@ -0,0 +1,65 @@
+#!/usr/bin/env python
+"""Simple plugin to show how to build new plugins for c-lightning
+
+It demonstrates how a plugin communicates with c-lightning, how it registers
+command line arguments that should be passed through and how it can register
+JSON-RPC commands. We communicate with the main daemon through STDIN and STDOUT,
+reading and writing JSON-RPC requests.
+
+"""
+import json
+import sys
+
+
+def json_hello(request):
+    greeting = "Hello {}".format(request['params']['name'])
+    return greeting
+
+
+def json_init(request):
+    return {
+        "options": [
+            {"name": "greeting", "type": "string", "default": "World"},
+        ],
+        "rpcmethods": [
+            {
+                "name": "hello",
+                "description": "Returns a personalized greeting for {name}",
+            },
+        ]
+    }
+
+
+def json_configure(request):
+    """The main daemon is telling us the relevant cli options
+    """
+    return None
+
+
+def json_ping(request):
+    return "pong"
+
+
+methods = {
+    'hello': json_hello,
+    'init': json_init,
+    'ping': json_ping,
+}
+
+
+partial = ""
+for l in sys.stdin:
+    partial += l
+    try:
+        request = json.loads(partial)
+        result = {
+            "jsonrpc": "2.0",
+            "result": methods[request['method']](request),
+            "id": request['id']
+        }
+        json.dump(result, fp=sys.stdout)
+        sys.stdout.write('\n')
+        sys.stdout.flush()
+        partial = ""
+    except Exception:
+        pass