Replace the date format option with a locale option

[Tyrrrz]

Closes #879

[codecov]

Codecov Report

Merging #1130 (b9103d2) into master (ccdf082) will increase coverage by 0.09%.
The diff coverage is 85.45%.

@@            Coverage Diff             @@
##           master    #1130      +/-   ##
==========================================
+ Coverage   69.01%   69.11%   +0.09%     
==========================================
  Files         107      106       -1     
  Lines        3896     3905       +9     
  Branches      627      631       +4     
==========================================
+ Hits         2689     2699      +10     
+ Misses        979      977       -2     
- Partials      228      229       +1     

Files Changed	Coverage Δ
...xporter.Core/Exporting/PlainTextMarkdownVisitor.cs	62.50% <0.00%> (+2.97%)	⬆️
...hatExporter.Core/Exporting/PreambleTemplate.cshtml	85.85% <0.00%> (ø)
...xporter.Core/Exporting/MessageGroupTemplate.cshtml	65.27% <57.14%> (-0.31%)	⬇️
...atExporter.Core/Markdown/Parsing/MarkdownParser.cs	93.38% <66.66%> (-0.61%)	⬇️
...rdChatExporter.Core/Exporting/JsonMessageWriter.cs	75.25% <92.30%> (+0.17%)	⬆️
...hatExporter.Cli/Commands/Base/ExportCommandBase.cs	77.45% <100.00%> (+0.39%)	⬆️
...dChatExporter.Core/Discord/Data/Common/ImageCdn.cs	53.84% <100.00%> (+6.02%)	⬆️
...iscordChatExporter.Core/Exporting/ExportContext.cs	88.52% <100.00%> (+3.90%)	⬆️
...iscordChatExporter.Core/Exporting/ExportRequest.cs	55.04% <100.00%> (+2.16%)	⬆️
...ChatExporter.Core/Exporting/HtmlMarkdownVisitor.cs	83.92% <100.00%> (ø)
... and 2 more

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