Add interactive prompts for selecting values

* Adds SelectionPrompt
* Adds MultiSelectionPrompt

Closes #210

src/Spectre.Console/Extensions/Int32Extensions.cs

@@ -0,0 +1,20 @@
+namespace Spectre.Console
+{
+    internal static class Int32Extensions
+    {
+        public static int Clamp(this int value, int min, int max)
+        {
+            if (value <= min)
+            {
+                return min;
+            }
+
+            if (value >= max)
+            {
+                return max;
+            }
+
+            return value;
+        }
+    }
+}