refactor: context formatter

[yolopunk]

Checklist

• [x] npm test passes
• [x] tests and/or benchmarks are included
• [x] documentation is changed or added
• [x] commit message follows commit guidelines

Affected core subsystem(s)

Description of change

我本来是想让contextFormatter输出在控制台的内容也可以显示颜色的，发现要改部分原有测试用例，就放弃了。现在只是整理了一下这部分的代码

[codecov[bot]]

Codecov Report

Merging #54 (e189ea9) into master (a539ffe) will increase coverage by 0.00%. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master      #54   +/-   ##
=======================================
  Coverage   99.45%   99.46%           
=======================================
  Files          14       14           
  Lines         370      372    +2     
=======================================
+ Hits          368      370    +2     
  Misses          2        2           

Impacted Files	Coverage Δ
lib/egg/context_logger.js	100.00% <100.00%> (ø)
lib/utils.js	100.00% <100.00%> (ø)

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