Feature: Extra arg detection

- Commands that do not accept filters or modifications now generate an error
  when extra arguments are specified.

src/commands/CmdExport.cpp

@@ -56,6 +56,9 @@ int CmdExport::execute (std::string& output)
   std::vector <Task> filtered;
   filter.subset (filtered, false);
 
+  if (filter.hasModifications ())
+    throw std::string (STRING_ERROR_NO_MODS);
+
   // Obey 'limit:N'.
   int rows = 0;
   int lines = 0;