Add yaml config file for exporter

[thepalbi]

This PR adds a new yaml config file, to use instead of CLI arguments. A sample file looks like this:

general:
  eth_provider_url: <redacted url>
  server_url: localhost:9368
  start_block_number: 0
targets:
  erc20:
    - name: "grafana token"
      contract: 0x83a7a21bec7c1b984c0056b5a3cb6fc76cace839
  wallet:
    address: 0x27084231Ff4077056bbB3e94A5B321ac0DB6d9e