Add JSON message writer

Closes #103

DiscordChatExporter.Core.Models/ExportFormat.cs

@@ -5,6 +5,7 @@
         PlainText,
         HtmlDark,
         HtmlLight,
-        Csv
+        Csv,
+        Json
     }
 }

DiscordChatExporter.Core.Models/Extensions.cs

@@ -18,6 +18,7 @@ namespace DiscordChatExporter.Core.Models
                 ExportFormat.HtmlDark => "html",
                 ExportFormat.HtmlLight => "html",
                 ExportFormat.Csv => "csv",
+                ExportFormat.Json => "json",
                 _ => throw new ArgumentOutOfRangeException(nameof(format))
             };
 
@@ -28,6 +29,7 @@ namespace DiscordChatExporter.Core.Models
                 ExportFormat.HtmlDark => "HTML (Dark)",
                 ExportFormat.HtmlLight => "HTML (Light)",
                 ExportFormat.Csv => "Comma Separated Values (CSV)",
+                ExportFormat.Json => "JavaScript Object Notation (JSON)",
                 _ => throw new ArgumentOutOfRangeException(nameof(format))
             };
     }