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

db/migrations/1528317902_create_token_supply_table.down.sql

@@ -0,0 +1 @@
+DROP TABLE token_supply;