Add go module support

[zhouzhuojie]

Fix #201

• Bump Go with the minimum version of 1.12
• Replace gometalinter with golangci-lint
• Replace retool with gobin

[codecov-io]

Codecov Report

Merging #248 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #248   +/-   ##
=======================================
  Coverage   82.27%   82.27%           
=======================================
  Files          26       26           
  Lines        1518     1518           
=======================================
  Hits         1249     1249           
  Misses        202      202           
  Partials       67       67

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