plugin: Register plugin cli options

We also make `--help` a non-early arg so it allows for the plugins to
register their options before printing the help message. The options
themselves are stored in a separate struct inbetween them being
registered and them being forwarded to the plugin. Currently only
supports string options.

Signed-off-by: Christian Decker <@cdecker>

contrib/helloworld-plugin/main.py

@@ -19,7 +19,10 @@ def json_hello(request):
 def json_init(request):
     return {
         "options": [
-            {"name": "greeting", "type": "string", "default": "World"},
+            {"name": "greeting",
+             "type": "string",
+             "default": "World",
+             "description": "What name should I call you?"},
         ],
         "rpcmethods": [
             {