Add conjured items

Conjured items degrade twice as fast as normal ones. Matched by name
prefix so any "Conjured ..." item works, not just the Mana Cake.
Wrote the tests first, then the updater. Only the Mana Cake lines
changed in the approved output.

python/gilded_rose.py

@@ -3,6 +3,7 @@
 AGED_BRIE = "Aged Brie"
 BACKSTAGE_PASS = "Backstage passes to a TAFKAL80ETC concert"
 SULFURAS = "Sulfuras, Hand of Ragnaros"
+CONJURED_PREFIX = "Conjured"
 
 MIN_QUALITY = 0
 MAX_QUALITY = 50
@@ -71,17 +72,29 @@ class SulfurasUpdater(ItemUpdater):
         pass
 
 
+class ConjuredUpdater(ItemUpdater):
+    """Degrades twice as fast as an ordinary item."""
+
+    def _quality_delta(self, item):
+        return 2 * super()._quality_delta(item)
+
+
 _UPDATERS_BY_NAME = {
     AGED_BRIE: AgedBrieUpdater(),
     BACKSTAGE_PASS: BackstagePassUpdater(),
     SULFURAS: SulfurasUpdater(),
 }
+_CONJURED_UPDATER = ConjuredUpdater()
 _DEFAULT_UPDATER = ItemUpdater()
 
 
 def updater_for(item):
     """Pick the rule set that applies to an item. Updaters are stateless."""
-    return _UPDATERS_BY_NAME.get(item.name, _DEFAULT_UPDATER)
+    if item.name in _UPDATERS_BY_NAME:
+        return _UPDATERS_BY_NAME[item.name]
+    if item.name.startswith(CONJURED_PREFIX):
+        return _CONJURED_UPDATER
+    return _DEFAULT_UPDATER
 
 
 class Item: