feat(excel): add some extra styling & width options for Excel export

[ghiscoding]

code referenced from another fork in this particular commit

• add the following excelExportOptions
  • customColumnWidth
  • columnHeaderStyle
• add the following Column options
  • exportColumnWidth
  • exportCustomGroupTotalsFormatter

[codecov]

Codecov Report

Merging #403 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff           @@
##           master   #403    +/-   ##
======================================
  Coverage     100%   100%            
======================================
  Files         141    141            
  Lines        8820   8840    +20     
  Branches     3040   2925   -115     
======================================
+ Hits         8820   8840    +20

Impacted Files	Coverage Δ
.../angular-slickgrid/services/excelExport.service.ts	100% <100%> (ø)	⬆️

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