Add SiteAdmin property to user

Octokit/GitHubModels.cs

@@ -1,7 +1,6 @@
 using System;
 using System.Diagnostics.CodeAnalysis;
 using System.Text;
-using System.Text.RegularExpressions;
 using System.Threading.Tasks;
 using Octokit.Http;
 
@@ -266,6 +265,11 @@ namespace Octokit
         /// The api URL for this user.
         /// </summary>
         public string Url { get; set; }
+
+        /// <summary>
+        /// Whether or not the user is an administrator of the site
+        /// </summary>
+        public bool SiteAdmin { get; set; }
     }
 
     public class Organization