Add ERC20 token watcher example

- starting with the totalSupply function
- sets contract config on transformer by passing it into the transformer
initializer
- handles block records with the same number for different nodes for both creating token_supply records, and finding missing blocks

examples/erc20_watcher/config.go

@@ -0,0 +1,33 @@
+// Copyright 2018 Vulcanize
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package erc20_watcher
+
+import "github.com/vulcanize/vulcanizedb/examples/constants"
+
+type ContractConfig struct {
+	Address    string
+	Abi        string
+	FirstBlock int64
+	LastBlock  int64
+	Name       string
+}
+
+var DaiConfig = ContractConfig{
+	Address:    constants.DaiContractAddress,
+	Abi:        constants.DaiAbiString,
+	FirstBlock: int64(4752008),
+	LastBlock:  -1,
+	Name:       "Dai",
+}