Implement Transfigured Assassin's Knife

Original:
```
Your Secondary has a 30% chance of dealing an additional 2 blows, dealing 50 damage each, in a radius around your target enemy.
```

Ideas:
```
When you use an ability or loot item with a cooldown, your next Secondary deals an additional 2 blows, dealing 50 damage each, in a radius around your target enemy.

When a % chance succeeds, your next Secondary deals an additional 2 blows, dealing 50 damage each, in a radius around your target enemy.
```

See https://github.com/Hejsil/TransfiguredLoot/issues/445