Load config with viper

[jeddie-yhc]

• add new directory configs/
• load erc20 token from configs/erc20.yaml
• start indexer from configs/config.yaml if the file exists

[codecov-io]

Codecov Report

Merging #13 into develop will not change coverage. The diff coverage is 100%.

@@           Coverage Diff            @@
##           develop      #13   +/-   ##
========================================
  Coverage    74.35%   74.35%           
========================================
  Files           14       14           
  Lines          858      858           
========================================
  Hits           638      638           
  Misses         180      180           
  Partials        40       40

Impacted Files	Coverage Δ
service/indexer/indexer.go	88.18% <100%> (ø)	:arrow_up:

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