Added addImplicitRule method

[codetestmg]

[codetestmg]

I think better solution to modify extend method in index.d.ts by passing third param

export function extend(name: string, callback: any, isImplicit: boolean): void;

[codecov[bot]]

Codecov Report

Merging #70 (82a8b7b) into master (8106ba5) will decrease coverage by 0.10%. The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master      #70      +/-   ##
==========================================
- Coverage   97.93%   97.83%   -0.11%     
==========================================
  Files          90       90              
  Lines         969      970       +1     
==========================================
  Hits          949      949              
- Misses         20       21       +1     

Impacted Files	Coverage Δ
lib/index.js	93.33% <0.00%> (-3.22%)	:arrow_down:

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