Add support for default choice in selection prompt

Closes #234

src/Spectre.Console/Widgets/Prompt/MultiSelectionPromptExtensions.cs

@@ -44,6 +44,73 @@ namespace Spectre.Console
             return obj;
         }
 
+        /// <summary>
+        /// Marks an item as selected.
+        /// </summary>
+        /// <typeparam name="T">The prompt result type.</typeparam>
+        /// <param name="obj">The prompt.</param>
+        /// <param name="index">The index of the item to select.</param>
+        /// <returns>The same instance so that multiple calls can be chained.</returns>
+        public static MultiSelectionPrompt<T> Select<T>(this MultiSelectionPrompt<T> obj, int index)
+        {
+            if (obj is null)
+            {
+                throw new ArgumentNullException(nameof(obj));
+            }
+
+            if (index < 0)
+            {
+                throw new ArgumentException("Index must be greater than zero", nameof(index));
+            }
+
+            obj.Selected.Add(index);
+            return obj;
+        }
+
+        /// <summary>
+        /// Marks multiple items as selected.
+        /// </summary>
+        /// <typeparam name="T">The prompt result type.</typeparam>
+        /// <param name="obj">The prompt.</param>
+        /// <param name="indices">The indices of the items to select.</param>
+        /// <returns>The same instance so that multiple calls can be chained.</returns>
+        public static MultiSelectionPrompt<T> Select<T>(this MultiSelectionPrompt<T> obj, params int[] indices)
+        {
+            if (obj is null)
+            {
+                throw new ArgumentNullException(nameof(obj));
+            }
+
+            foreach (var index in indices)
+            {
+                Select(obj, index);
+            }
+
+            return obj;
+        }
+
+        /// <summary>
+        /// Marks multiple items as selected.
+        /// </summary>
+        /// <typeparam name="T">The prompt result type.</typeparam>
+        /// <param name="obj">The prompt.</param>
+        /// <param name="indices">The indices of the items to select.</param>
+        /// <returns>The same instance so that multiple calls can be chained.</returns>
+        public static MultiSelectionPrompt<T> Select<T>(this MultiSelectionPrompt<T> obj, IEnumerable<int> indices)
+        {
+            if (obj is null)
+            {
+                throw new ArgumentNullException(nameof(obj));
+            }
+
+            foreach (var index in indices)
+            {
+                Select(obj, index);
+            }
+
+            return obj;
+        }
+
         /// <summary>
         /// Adds multiple choices.
         /// </summary>
@@ -179,4 +246,4 @@ namespace Spectre.Console
             return obj;
         }
     }
-}
+}