Add method to write VT/ANSI control codes

Adds a new extension method IAnsiConsole.WriteAnsi that writes
VT/ANSI control codes to the console. If VT/ANSI control codes are
not supported, nothing will be written.

src/Spectre.Console/Extensions/Advanced/AnsiConsoleExtensions.cs

@@ -0,0 +1,28 @@
+using System;
+
+namespace Spectre.Console.Advanced
+{
+    /// <summary>
+    /// Contains extension methods for <see cref="IAnsiConsole"/>.
+    /// </summary>
+    public static class AnsiConsoleExtensions
+    {
+        /// <summary>
+        /// Writes a VT/Ansi control code sequence to the console (if supported).
+        /// </summary>
+        /// <param name="console">The console to write to.</param>
+        /// <param name="sequence">The VT/Ansi control code sequence to write.</param>
+        public static void WriteAnsi(this IAnsiConsole console, string sequence)
+        {
+            if (console is null)
+            {
+                throw new ArgumentNullException(nameof(console));
+            }
+
+            if (console.Profile.Capabilities.Ansi)
+            {
+                console.Write(new ControlCode(sequence));
+            }
+        }
+    }
+}

src/Spectre.Console/Extensions/AnsiConsoleExtensions.cs

@@ -38,6 +38,11 @@ namespace Spectre.Console
         /// <param name="text">The text to write.</param>
         public static void Write(this IAnsiConsole console, string text)
         {
+            if (console is null)
+            {
+                throw new ArgumentNullException(nameof(console));
+            }
+
             console.Write(new Text(text, Style.Plain));
         }
 
@@ -49,6 +54,11 @@ namespace Spectre.Console
         /// <param name="style">The text style.</param>
         public static void Write(this IAnsiConsole console, string text, Style style)
         {
+            if (console is null)
+            {
+                throw new ArgumentNullException(nameof(console));
+            }
+
             console.Write(new Text(text, style));
         }