Added support for mailgun variables

Author: belidzs

src/FluentEmail.Core/Email.cs

@@ -484,6 +484,19 @@ public IFluentEmail Tag(string tag)
             return this;
         }
 
+        /// <summary>
+        /// Adds variables to the Email. This is currently only supported by the Mailgun provider. <see href="https://documentation.mailgun.com/en/latest/user_manual.html#attaching-data-to-messages"/>
+        /// </summary>
+        /// <param name="key">Variable name</param>
+        /// /// <param name="value">Variable value</param>
+        /// <returns>Instance of the Email class</returns>
+        public IFluentEmail Variable(string key, string value)
+        {
+            Data.Variables.Add(key, value);
+
+            return this;
+        }
+
         public IFluentEmail Header(string header, string body)
         {
             Data.Headers.Add(header, body);

src/FluentEmail.Core/IFluentEmail.cs

@@ -234,6 +234,14 @@ public interface IFluentEmail: IHideObjectMembers
 	    /// <returns>Instance of the Email class</returns>
 	    IFluentEmail Tag(string tag);
 
+		/// <summary>
+		/// Adds variables to the Email. This is currently only supported by the Mailgun provider. <see href="https://documentation.mailgun.com/en/latest/user_manual.html#attaching-data-to-messages"/>
+		/// </summary>
+		/// <param name="key">Variable name</param>
+		/// /// <param name="value">Variable value</param>
+		/// <returns>Instance of the Email class</returns>
+		IFluentEmail Variable(string key, string value);
+
 	    /// <summary>
 	    /// Adds header to the Email.
 	    /// </summary>

src/FluentEmail.Core/Models/EmailData.cs

@@ -14,7 +14,8 @@ public class EmailData
         public string Body { get; set; }
         public string PlaintextAlternativeBody { get; set; }
         public Priority Priority { get; set; }
-        public IList<string> Tags { get; set; }
+        public List<string> Tags { get; set; }
+        public Dictionary<string, string> Variables { get; set; }
 
         public bool IsHtml { get; set; }
         public IDictionary<string, string> Headers { get; set; }
@@ -27,6 +28,7 @@ public EmailData()
             ReplyToAddresses = new List<Address>();
             Attachments = new List<Attachment>();
             Tags = new List<string>();
+            Variables = new Dictionary<string, string>();
             Headers = new Dictionary<string, string>();
         }
     }

src/Senders/FluentEmail.Mailgun/MailgunSender.cs

@@ -80,6 +80,11 @@ public async Task<SendResponse> SendAsync(IFluentEmail email, CancellationToken?
                 parameters.Add(new KeyValuePair<string, string>("o:tag", x));
             });
 
+            foreach (var item in email.Data.Variables)
+            {
+                parameters.Add(new KeyValuePair<string, string>($"v:{item.Key}", item.Value));
+            }
+
             foreach (var emailHeader in email.Data.Headers)
             {
                 var key = emailHeader.Key;