countdown: add "recurring until" support

tacking onto the recurring option, this allows for having the
recurringness end after a while.

closes bss/dr.botzo#31

countdown/migrations/0005_countdownitem_recurring_until.py

@@ -0,0 +1,20 @@
+# -*- coding: utf-8 -*-
+# Generated by Django 1.10.5 on 2017-02-24 01:12
+from __future__ import unicode_literals
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('countdown', '0004_countdownitem_recurring_period'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='countdownitem',
+            name='recurring_until',
+            field=models.DateTimeField(blank=True, default=None, null=True),
+        ),
+    ]