customParseFormat: Add case insensitive search for `MMM` and `MMMM` format

[eliandoran]

Parsing date inputs such as 21-December-18 (MMMM) and 2018 Feb 03 (MMM) works just fine, however as soon as the user enters inputs such as 21-december-18 or 2018 Feb 03, the date will not be parsed. This is especially the case for languages where it is not customary to write months with an upper case letter.

This PR implements a method which searches through the months with case insensitive.

Motivation for this PR: Was working on a application which parses user dates, but it would fail when users typed months using only small letters.

Initially wanted to implement case insensitive search only if strict mode is off, however it appears that case insensitivity is already handled in strict mode as well (see recognizes midnight in small letters test).

[codecov[bot]]

Codecov Report

Merging #1769 (6f57e67) into dev (4a7b7d0) will not change coverage. The diff coverage is 100.00%.

@@            Coverage Diff            @@
##               dev     #1769   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          179       179           
  Lines         1996      2002    +6     
  Branches       507       508    +1     
=========================================
+ Hits          1996      2002    +6     

Impacted Files	Coverage Δ
src/plugin/customParseFormat/index.js	100.00% <100.00%> (ø)

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[BePo65]

fixes issue #1720