Add Pit file stability fee event

- file event on the pit contract is overloaded and each implementation
  logs a note
- existing Pit file transformer now specified as dealing with ilks

pkg/transformers/pit_file/config.go

@@ -7,7 +7,7 @@ import (
 var PitFileConfig = shared.TransformerConfig{
 	ContractAddress:     "0xff3f2400f1600f3f493a9a92704a29b96795af1a", // temporary address from Ganache deploy
 	ContractAbi:         shared.PitABI,
-	Topics:              []string{shared.PitFileSignatureOne},
+	Topics:              []string{shared.PitFileIlkSignature, shared.PitFileDebtCeilingSignature, shared.PitFileStabilityFeeSignature},
 	StartingBlockNumber: 0,
 	EndingBlockNumber:   100,
 }