Add DeleteFunc

This shares DeletePrefixs's implementation.

cache.go

@@ -63,6 +63,15 @@ func (c *Cache) DeletePrefix(prefix string) int {
 	return count
 }
 
+// Deletes all items that the matches func evaluates to true.
+func (c *Cache) DeleteFunc(matches func(key string, item interface{}) bool) int {
+	count := 0
+	for _, b := range c.buckets {
+		count += b.deleteFunc(matches, c.deletables)
+	}
+	return count
+}
+
 // Get an item from the cache. Returns nil if the item wasn't found.
 // This can return an expired item. Use item.Expired() to see if the item
 // is expired and item.TTL() to see how long until the item expires (which