Enhancement - _commands

- Added undocumented _commands command to support completion scripts.

src/Context.cpp

@@ -198,6 +198,7 @@ std::string Context::dispatch ()
   else if (cmd.command == "undo")          {       handleUndo               (); }
   else if (cmd.command == "_projects")     { out = handleCompletionProjects (); }
   else if (cmd.command == "_tags")         { out = handleCompletionTags     (); }
+  else if (cmd.command == "_commands")     { out = handleCompletionCommands (); }
   else if (cmd.command == "" &&
            sequence.size ())               { out = handleModify             (); }