implement plugin command execution

- change .proto file for plugins
- add global usage guide for all commands
- handle commands not included in the built-in set

protobuf/plugin.proto

@@ -18,6 +18,7 @@ message PluginCommandListRequest {}
 
 message PluginExecuteRequest {
   string command_id = 1;
+  string issuer_id = 2;
   repeated string arg_vector = 5;
 }
 
@@ -26,7 +27,7 @@ message Response {
   oneof res {
     PluginInitializeResponse initialize_res = 10;
     PluginCommandListResponse command_list_res = 11;
-    PluginExecuteResponse execute_res = 12;
+    CommandReply cmd_reply = 12;
   }
 }
 
@@ -48,6 +49,10 @@ message CommandSpec {
   string description = 4;
 }
 
-message PluginExecuteResponse {
-  
+message CommandReply {
+  bool edit = 1;
+  oneof reply {
+    string text = 2;
+    bool end = 8;
+  }
 }