Added a way to add 'implicit' custom rules to the Validator

[DanielAlt]

I opened an issue describing why. Issue #46

LMK

[codecov]

Codecov Report

Merging #47 (e7c177f) into master (f2db06a) will decrease coverage by 0.10%.
The diff coverage is 50.00%.

@@            Coverage Diff             @@
##           master      #47      +/-   ##
==========================================
- Coverage   98.12%   98.02%   -0.11%     
==========================================
  Files          89       89              
  Lines         960      962       +2     
==========================================
+ Hits          942      943       +1     
- Misses         18       19       +1     

Impacted Files	Coverage Δ
lib/validator.js	97.31% <0.00%> (-0.53%)	⬇️
lib/index.js	96.42% <100.00%> (+0.13%)	⬆️

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