Add -r flag to tag subcommand to enforce relative dates

Relative dates were already interpreted for due and start tags, but you
can also enforce it for other tags by passing the -r flag to the tag
command.

test/test_tag_command.py

@@ -173,6 +173,35 @@ class TagCommandTest(CommandTest):
         self.assertEqual(self.output, "|  3| Baz due:2014-10-20 foo:today\n")
         self.assertEqual(self.errors, "")
 
+    @freeze_time('2017, 1, 12')
+    def test_set_tag13(self):
+        """
+        Convert relative dates when forced to.
+        """
+
+        command = TagCommand(["-r", "3", "foo", "today"], self.todolist,
+                             self.out, self.error)
+
+        command.execute()
+
+        self.assertTrue(self.todolist.dirty)
+        self.assertEqual(self.output, "|  3| Baz due:2014-10-20 foo:2017-01-12\n")
+        self.assertEqual(self.errors, "")
+
+    def test_set_tag14(self):
+        """
+        Leave the original value when an invalid relative date was given.
+        """
+
+        command = TagCommand(["-r", "3", "foo", "bar"], self.todolist,
+                             self.out, self.error)
+
+        command.execute()
+
+        self.assertTrue(self.todolist.dirty)
+        self.assertEqual(self.output, "|  3| Baz due:2014-10-20 foo:bar\n")
+        self.assertEqual(self.errors, "")
+
     def test_rm_tag01(self):
         command = TagCommand(["1", "due"], self.todolist, self.out, self.error)
         command.execute()

topydo/commands/TagCommand.py

@@ -31,6 +31,7 @@ class TagCommand(Command):
 
         self.force = False
         self.force_add = False
+        self.relative_date = False
         self.todo = None
         self.tag = None
         self.value = None
@@ -38,12 +39,14 @@ class TagCommand(Command):
         self.current_values = []
 
     def _process_flags(self):
-        flags, args = self.getopt("af")
+        flags, args = self.getopt("afr")
         for flag, _ in flags:
             if flag == "-a":
                 self.force_add = True
             elif flag == "-f":
                 self.force = True
+            elif flag == "-r":
+                self.relative_date = True
 
         self.args = args
 
@@ -93,7 +96,10 @@ class TagCommand(Command):
         return answer
 
     def _convert_relative_dates(self):
-        if self.tag == config().tag_start() or self.tag == config().tag_due():
+        is_start_tag = self.tag == config().tag_start()
+        is_due_tag = self.tag == config().tag_due()
+
+        if self.relative_date or is_start_tag or is_due_tag:
             real_date = relative_date_to_date(self.value)
 
             if real_date:
@@ -143,5 +149,7 @@ is omitted, the TAG is removed from the todo item.
 -a : Do not change the current value of the TAG if it exists, but add a new
      VALUE for the given TAG.
 -f : Force setting/removing all values of the TAG. Prevents interaction with
-     the user.\
+     the user.
+-r : Interpret the given value as a relative date and convert it to an absolute
+     date.\
 """