Add panel header support

Closes #63

src/Spectre.Console/Rendering/PanelExtensions.cs

@@ -0,0 +1,50 @@
+using System;
+
+namespace Spectre.Console.Rendering
+{
+    /// <summary>
+    /// Contains extension methods for <see cref="Panel"/>.
+    /// </summary>
+    public static class PanelExtensions
+    {
+        /// <summary>
+        /// Sets the panel header.
+        /// </summary>
+        /// <param name="panel">The panel.</param>
+        /// <param name="text">The header text.</param>
+        /// <param name="style">The header style.</param>
+        /// <param name="alignment">The header alignment.</param>
+        /// <returns>The same instance so that multiple calls can be chained.</returns>
+        public static Panel SetHeader(this Panel panel, string text, Style? style = null, Justify? alignment = null)
+        {
+            if (panel is null)
+            {
+                throw new ArgumentNullException(nameof(panel));
+            }
+
+            if (text is null)
+            {
+                throw new ArgumentNullException(nameof(text));
+            }
+
+            return SetHeader(panel, new Header(text, style, alignment));
+        }
+
+        /// <summary>
+        /// Sets the panel header.
+        /// </summary>
+        /// <param name="panel">The panel.</param>
+        /// <param name="header">The header to use.</param>
+        /// <returns>The same instance so that multiple calls can be chained.</returns>
+        public static Panel SetHeader(this Panel panel, Header header)
+        {
+            if (panel is null)
+            {
+                throw new ArgumentNullException(nameof(panel));
+            }
+
+            panel.Header = header;
+            return panel;
+        }
+    }
+}