'postpone' can now handle multiple todos

topydo will report which todo IDs ('numbers') were invalid if user
supplied more than one on command-line. Error for operation on single
todo ID are reported as usual (for consistency with other commands sake
and because I'm lazy and don't want to fix 2 tests :P).

topydo/lib/PostponeCommand.py

@@ -58,10 +58,28 @@ class PostponeCommand(Command):
 
         self._process_flags()
 
+        todos = []
+        invalid_numbers = []
+        for number in self.args[:-1]:
             try:
-            todo = self.todolist.todo(self.argument(0))
-            pattern = self.argument(1)
+                todos.append(self.todolist.todo(number))
+            except InvalidTodoException:
+                invalid_numbers.append(number)
+
+        if len(invalid_numbers) > 0 and len(todos) > 0:
+            for number in invalid_numbers:
+                self.error("Invalid todo number given: {}.".format(number))
+        elif len(invalid_numbers) == 1 and len(todos) == 0:
+            self.error("Invalid todo number given.")
+        else:
+
+            try:
+                pattern = self.args[-1]
+
+                self.printer.add_filter(PrettyPrinterNumbers(self.todolist))
 
+                if len(todos) > 0:
+                    for todo in todos:
                         offset = _get_offset(todo)
                         new_due = relative_date_to_date(pattern, offset)
 
@@ -74,15 +92,15 @@ class PostponeCommand(Command):
                             todo.set_tag(config().tag_due(), new_due.isoformat())
 
                             self.todolist.set_dirty()
-                self.printer.add_filter(PrettyPrinterNumbers(self.todolist))
                             self.out(self.printer.print_todo(todo))
                         else:
                             self.error("Invalid date pattern given.")
+                            break
+                else:
+                    self.error(self.usage())
 
-        except InvalidCommandArgument:
+            except (InvalidCommandArgument, IndexError):
                 self.error(self.usage())
-        except (InvalidTodoException):
-            self.error("Invalid todo number given.")
 
     def usage(self):
         return "Synopsis: postpone [-s] <NUMBER> <PATTERN>"