add NewCustomized() so that client works with etherscan-family API like BscScan

[nanmu42]

This PR replaces #23 .

Thanks for Kevin J. Qiu(#23) for the idea.

[codecov]

Codecov Report

Merging #24 (c453d3c) into master (3ddd9ca) will increase coverage by 0.69%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #24      +/-   ##
==========================================
+ Coverage   75.84%   76.54%   +0.69%     
==========================================
  Files          10       10              
  Lines         236      243       +7     
==========================================
+ Hits          179      186       +7     
  Misses         34       34              
  Partials       23       23              

Impacted Files	Coverage Δ
client.go	55.55% <100.00%> (+4.20%)	⬆️

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