Allow the contains filter to also search on embed content

Closes #670

DiscordChatExporter.Core/Exporting/Filtering/ContainsMessageFilter.cs

@@ -1,4 +1,5 @@
-using System.Text.RegularExpressions;
+using System.Linq;
+using System.Text.RegularExpressions;
 using DiscordChatExporter.Core.Discord.Data;
 
 namespace DiscordChatExporter.Core.Exporting.Filtering
@@ -9,10 +10,25 @@ namespace DiscordChatExporter.Core.Exporting.Filtering
 
         public ContainsMessageFilter(string text) => _text = text;
 
-        public override bool Filter(Message message) => Regex.IsMatch(
-            message.Content,
-            "\\b" + _text + "\\b",
-            RegexOptions.IgnoreCase | RegexOptions.CultureInvariant
-        );
+        private bool Filter(string? content) =>
+            !string.IsNullOrWhiteSpace(content) &&
+            Regex.IsMatch(
+                content,
+                "\\b" + Regex.Escape(_text) + "\\b",
+                RegexOptions.IgnoreCase | RegexOptions.CultureInvariant
+            );
+
+        public override bool Filter(Message message) =>
+            Filter(message.Content) ||
+            message.Embeds.Any(e =>
+                Filter(e.Title) ||
+                Filter(e.Author?.Name) ||
+                Filter(e.Description) ||
+                Filter(e.Footer?.Text) ||
+                e.Fields.Any(f =>
+                    Filter(f.Name) ||
+                    Filter(f.Value)
+                )
+            );
     }
 }