pylightning: Warn users of plugins that may break due to extra args

We recently noticed that the way we unpack the call arguments for hooks and
notifications in pylightning breaks pretty quickly once you start changing the
hook and notification params. If you add params they will not get mapped
correctly causing the plugin to error out.

This can be fixed by adding a `VAR_KEYWORD` argument to the calbacks, i.e., by
adding a single `**kwargs` argument at the end of the signature. This commit
adds a check that such a catch-all argument exists, and emits a warning if it
doesn't.

It also fixes up the plugins that we ship ourselves.

Signed-off-by: Christian Decker <decker.christian@gmail.com>

tests/plugins/reject.py

@@ -13,7 +13,7 @@ plugin = Plugin()
 
 
 @plugin.hook('peer_connected')
-def on_connected(peer, plugin):
+def on_connected(peer, plugin, **kwargs):
     if peer['id'] in plugin.reject_ids:
         print("{} is in reject list, disconnecting".format(peer['id']))
         return {'result': 'disconnect', 'error_message': 'You are in reject list'}