Refactor default file name generation and add date ranges there

Closes #29

DiscordChatExporter.Core/Helpers/ExportHelper.cs

@@ -0,0 +1,52 @@
+using System;
+using System.IO;
+using System.Text;
+using DiscordChatExporter.Core.Models;
+
+namespace DiscordChatExporter.Core.Helpers
+{
+    public static class ExportHelper
+    {
+        public static string GetDefaultExportFileName(ExportFormat format, Guild guild, Channel channel,
+            DateTime? from = null, DateTime? to = null)
+        {
+            var result = new StringBuilder();
+
+            // Append guild and channel names
+            result.Append($"{guild.Name} - {channel.Name}");
+
+            // Append date range
+            if (from != null || to != null)
+            {
+                result.Append(" (");
+
+                // Both 'from' and 'to' are set
+                if (from != null && to != null)
+                {
+                    result.Append($"{from:yyyy-MM-dd} to {to:yyyy-MM-dd}");
+                }
+                // Only 'from' is set
+                else if (from != null)
+                {
+                    result.Append($"after {from:yyyy-MM-dd}");
+                }
+                // Only 'to' is set
+                else
+                {
+                    result.Append($"before {to:yyyy-MM-dd}");
+                }
+
+                result.Append(")");
+            }
+
+            // Append extension
+            result.Append($".{format.GetFileExtension()}");
+
+            // Replace invalid chars
+            foreach (var invalidChar in Path.GetInvalidFileNameChars())
+                result.Replace(invalidChar, '_');
+
+            return result.ToString();
+        }
+    }
+}