fix(filter): Date Filters using Flatpickr throw error w/invalid locale

[ghiscoding]

• ref Angular-Slickgrid issue #346
• when using a locale that Flatpickr doesn't support, like "ua", it should use "en" (English) as default locale.
  • it will display a console warning that this locale is not supported but not throw an error (unless user set a different locale, see next)
  • user can choose to override the locale by setting locale to use via
    • filter: { filterOptions: { locale: 'en' } }

New behavior with this PR, it will now show a console warning to advise user that the locale is not supported, it will use English locale instead of throwing an error

[codecov]

Codecov Report

Merging #267 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #267      +/-   ##
==========================================
+ Coverage   99.83%   99.83%   +0.01%     
==========================================
  Files         150      150              
  Lines        8448     8456       +8     
  Branches     2784     2786       +2     
==========================================
+ Hits         8433     8441       +8     
  Misses         15       15

Impacted Files	Coverage Δ
src/aurelia-slickgrid/services/grid.service.ts	100% <100%> (ø)	⬆️
...rc/aurelia-slickgrid/filters/compoundDateFilter.ts	100% <100%> (ø)	⬆️
src/aurelia-slickgrid/filters/dateRangeFilter.ts	100% <100%> (ø)	⬆️

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