Streaming exporter

Fixes #125
Closes #177

DiscordChatExporter.Core.Rendering/CsvMessageRenderer.cs

@@ -0,0 +1,28 @@
+using System.Threading.Tasks;
+using DiscordChatExporter.Core.Models;
+using DiscordChatExporter.Core.Rendering.Logic;
+
+namespace DiscordChatExporter.Core.Rendering
+{
+    public class CsvMessageRenderer : MessageRendererBase
+    {
+        private bool _isHeaderRendered;
+
+        public CsvMessageRenderer(string filePath, RenderContext context)
+            : base(filePath, context)
+        {
+        }
+
+        public override async Task RenderMessageAsync(Message message)
+        {
+            // Render header if it's the first entry
+            if (!_isHeaderRendered)
+            {
+                await Writer.WriteLineAsync(CsvRenderingLogic.FormatHeader(Context));
+                _isHeaderRendered = true;
+            }
+
+            await Writer.WriteLineAsync(CsvRenderingLogic.FormatMessage(Context, message));
+        }
+    }
+}