Streaming exporter

Fixes #125
Closes #177

DiscordChatExporter.Core.Rendering/MessageRendererBase.cs

@@ -0,0 +1,23 @@
+using System.IO;
+using System.Threading.Tasks;
+using DiscordChatExporter.Core.Models;
+
+namespace DiscordChatExporter.Core.Rendering
+{
+    public abstract class MessageRendererBase : IMessageRenderer
+    {
+        protected TextWriter Writer { get; }
+
+        protected RenderContext Context { get; }
+
+        protected MessageRendererBase(string filePath, RenderContext context)
+        {
+            Writer = File.CreateText(filePath);
+            Context = context;
+        }
+
+        public abstract Task RenderMessageAsync(Message message);
+
+        public virtual ValueTask DisposeAsync() => Writer.DisposeAsync();
+    }
+}