Strip html tags (#28)

Add a helper for stripping html tags

CSharp/Microsoft.Bot.Schema.Teams/MessageActionsPayloadBodyExtensions.cs

@@ -0,0 +1,54 @@
+// <copyright file="MessageActionsPayloadBodyExtensions.cs" company="Microsoft">
+// Licensed under the MIT License.
+// </copyright>
+
+namespace Microsoft.Bot.Schema.Teams
+{
+    using System.Collections.Generic;
+    using HtmlAgilityPack;
+
+    /// <summary>
+    /// MessageActionsPayloadBody extensions.
+    /// </summary>
+
+    public static class MessageActionsPayloadBodyExtensions
+    {
+        private static readonly HashSet<string> textRestrictedHtmlTags = new HashSet<string> { "at", "attachment" };
+
+        /// <summary>
+        /// Strip HTML tags from MessageActionsPayloadBody content.
+        /// </summary>
+        /// <param name="body">The MessageActionsPayloadBody.</param>
+        /// <returns>Plain text content.</returns>
+        public static string GetPlainTextContent(this MessageActionsPayloadBody body)
+        {
+            var doc = new HtmlDocument();
+            doc.LoadHtml(body.content);
+            return StripHtmlTags(doc.DocumentNode, textRestrictedHtmlTags);
+        }
+
+        private static string StripHtmlTags(HtmlNode node, ISet<string> tags)
+        {
+            string result = "";
+            if (tags.Contains(node.Name))
+            {
+                result += node.OuterHtml;
+            }
+            else
+            {
+                foreach (HtmlNode childNode in node.ChildNodes)
+                {
+                    if (childNode.NodeType == HtmlNodeType.Text)
+                    {
+                        result += childNode.InnerText;
+                    }
+                    else
+                    {
+                        result += StripHtmlTags(childNode, tags);
+                    }
+                }
+            }
+            return result;
+        }                            
+    }
+}