Sanitize file names when exporting media

Fixes #332

DiscordChatExporter.Domain/Internal/PathEx.cs

@@ -0,0 +1,18 @@
+using System.IO;
+using System.Text;
+
+namespace DiscordChatExporter.Domain.Internal
+{
+    internal static class PathEx
+    {
+        public static StringBuilder EscapePath(StringBuilder pathBuffer)
+        {
+            foreach (var invalidChar in Path.GetInvalidFileNameChars())
+                pathBuffer.Replace(invalidChar, '_');
+
+            return pathBuffer;
+        }
+
+        public static string EscapePath(string path) => EscapePath(new StringBuilder(path)).ToString();
+    }
+}