Add onkey command for executing commands after reading a key

This completes the various user interaction primitives, on_next_key
was the last not to be available through a command.

README.asciidoc

@@ -1155,6 +1155,8 @@ Some helper commands can be used to define composite commands:
  * `:prompt <prompt> <register> <command>`: Prompt the user for a string, when
      the user validates, store the result in given <register> and run <commmand>.
      the -init <str> switch allows setting initial content. 
+ * `:onkey <register> <command>`: Wait for next key from user, writes it into given
+     <register> and execute commands.
  * `:menu <label1> <commands1> <label2> <commands2>...`: display a menu using
      labels, the selected label's commands are executed.
      `menu` can take a -auto-single argument, to automatically run commands

src/commands.cc

@@ -1300,6 +1300,26 @@ const CommandDesc menu_cmd = {
     }
 };
 
+const CommandDesc onkey_cmd = {
+    "onkey",
+    nullptr,
+    "onkey <register> <command>: wait for next user key, store it in <register> and execute <command>",
+    ParameterDesc{ {}, ParameterDesc::Flags::None, 2, 2 },
+    CommandFlags::None,
+    CommandHelper{},
+    CommandCompleter{},
+    [](const ParametersParser& parser, Context& context)
+    {
+        String reg = parser[0];
+        String command = parser[1];
+        context.input_handler().on_next_key(KeymapMode::None,
+                                            [=](Key key, Context& context) {
+            RegisterManager::instance()[reg] = key_to_str(key);
+            CommandManager::instance().execute(command, context);
+        });
+    }
+};
+
 const CommandDesc info_cmd = {
     "info",
     nullptr,
@@ -1517,6 +1537,7 @@ void register_commands()
     register_command(eval_string_cmd);
     register_command(prompt_cmd);
     register_command(menu_cmd);
+    register_command(onkey_cmd);
     register_command(info_cmd);
     register_command(try_catch_cmd);
     register_command(face_cmd);