refactor(isDate): add strictMode and prevent mixed delimiters

[tux-tn]

Hello,

I made some changes to isDate to allow for more flexibility.

• Add strictMode to enforce strict comparison between input and format. In strictMode Date objects, inputs with a different delimiter or string format are disallowed.
• Disallow validating inputs with mixed delimiters (Fixes Bug in Date validation, isDate() function #1395 )
• Allow passing custom delimiters list
• This PR is backward compatible with old format isDate(input, format)

Checklist

• PR contains only changes related; no stray files, etc.
• README updated (where applicable)
• Tests written (where applicable)

[codecov]

Codecov Report

Merging #1402 into master will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master     #1402   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           95        95           
  Lines         1248      1257    +9     
=========================================
+ Hits          1248      1257    +9     

Impacted Files	Coverage Δ
src/lib/isDate.js	100.00% <100.00%> (ø)

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[tux-tn]

strictModeis based on @ezkemboi idea in #1395 but with few changes. After some research, i can't find a standard date format with mixed delimiters and i don't think that allowing it in #1270 was purposely. For this reason strictMode intent here is to prevent validating inputs different from format.

validator.isDate('2020-05-02', { format: 'YYYY/MM/DD', strictMode: true}); /// returns false
validator.isDate(new Date(), { format: 'YYYY/MM/DD', strictMode: true}); // returns false

[ezkemboi]

@tux-tn thanks for working on this.
Cc. @profnandaa

[profnandaa]

LGTM, thanks for the good work on this. I'm okay at some point down the line, making strictMode = true the default -- but for now, this is okay to maintain b/c.